Signals....

Microwave transmitters were originally created to transmit radar signals in WWII. When a worker noticed that being near the transmitter melted a candy bar in his pocket, it led to the development of the microwaves that we use in our homes today. DeathSpiral321

Slinked.

Slinky was originally meant to be used as springs inside sensitive boat devices.

"Mechanical engineer Richard James invented the Slinky by accident. In 1943, he was working to devise springs that could keep sensitive ship equipment steady at sea. After accidentally knocking some samples off a shelf, he watched in amazement as they gracefully "walked" down instead of falling. Along with his wife Betty, James developed a plan to turn his invention into the next big novelty toy."

Source: National Toy Hall of Fame GuavaGila

Useless TV.

MTV, a.k.a. Music Televison. clofas1

And TLC, or "The Learning Channel." Not a lot of learning happening on that channel these days, unless you enjoy "learning" about the daily lives of morbidly obese people, and idiots marrying morons from different countries in a matter of weeks. HorseMeatSandwich

It's Magic. 

Hitachi magic wand. It was originally intended as a massager. MechanicalHorse

Hitachi still explicitly sells it as a massager and nothing else. It also gets upset at people claiming it's a sex toy (which is also why they have ceased putting their brandname on it).

Hitachi ceased production of the device in 2013 because of concerns about having the company name attached to a sex toy. Vibratex persuaded the company to continue manufacturing it under the name "Original Magic Wand", omitting the Hitachi name. In 2014, the company used the name "Magic Wand Original".

https://en.wikipedia.org/wiki/Hitachi_Magic_Wand That-General

So many Options..... 

Viagra. It was invented as a blood pressure medicine. Dilettante

It's still used to treat a form of high blood pressure called pulmonary hypertension in both men and women. It's sold under another name, Revatio, for marketing purposes. Both Viagra and Revatio are the same drug, sildenafil. MuffinMedic

In my Heel...

High-heeled shoes. They were originally for men and used for horse riding, as the heels were supposed to keep the feet more secure in the stirrups.

Now, they're just fancy women's shoes. Vyzantinist

Leave a message. 

Benadryl was developed as a sleep aid but it's antihistamine properties were better.

Also, and maybe this has been said, but the pound sign (#). Started as shorthand for "pound weight", then commonly used in the 80's for voicemail access, now it's used to light fires on social media. fergusoniv

zesting....

The microplane was initially developed as a wood rasp, but now it's used pretty much exclusively for zesting lemons and grating parmesan cheese. t_robthomast

Eek.....

Lysol. It was invented in the 1920s as a vaginal cleaner. It was marketed to married women to cure the "marital grief" caused by a stinky hoo-ha. This article from Smithsonian Magazine (https://www.smithsonianmag.com/smart-news/lysols-vintage-ads-subtly-pushed-women-to-use-its-disinfectant-as-birth-control-218734/) has great ads showing poor, beautiful, glamorous ladies whose husbands have totally lost interest in sex with them until they put Lysol up their vaginas. Eek. 1pennygadget

Clean Away....

WD-40 aka Water Displacement the 40th attempt. WD-40 was invented to keep the outer skin of missiles from rust and corrosion.

(heh, missile skin). HomegrownMO

Doe. Duh.

Playdough. Was supposed to be a wallpaper cleaner originally. Mysterion_x

No need paint people....

Paintball guns; they were originally called paintball markers and were invented as a way for loggers and park rangers to mark trees for trimming, cutting down, etc. without having to walk up to each tree with spray paint. A couple of dudes who were using them started shooting at each other for fun and paintball as a sport was born. In fact some people still call them 'markers' to avoid the stigma some associate with the word 'gun'. nyeark

Best of the Best.... 

The best man at a wedding was the best swordsman the groom could afford. If anyone objected to the wedding, the best man would duel him.

The honeymoon lasted a whole month after the wedding, where the bride's father would supply the groom with all the mead he could drink. proc89

Drink. 

Nalgene bottles - the plastic water bottles that are popular with hikers/campers/other outdoor activity folks were originally created for use as medical/scientific lab equipment as a lighter, shatterproof alternative to glass. VictorBlimpmuscle

Sneeze Away....

Kleenex. Its original purpose was to act as insulation in gas masks during World War I. After the war, its parent company advertised it as a way to remove makeup. Dilettante

​From Wikipedia.....

Aspartame was discovered in 1965 by James M. Schlatter, a chemist working for G.D. Searle & Company. Schlatter had synthesized aspartame as an intermediate step in generating a tetrapeptide of the hormone gastrin, for use in assessing an anti-ulcer drug candidate.[53] He discovered its sweet taste when he licked his finger, which had become contaminated with aspartame, to lift up a piece of paper.

Torunn Atteraas Garin participated in the development of aspartame as an artificial sweetener. Limp_Distribution

The Elixirs.... 

Maybe it's been said already:

Coca-cola and Dr. Pepper were originally marketed as medicine and health tonics. I believe Dr. Pepper's original name was something like "Dr. Pepper's health elixir." Extrovert3d

Super.....

Superglue, I think it was originally an instant suture in med kits for soldiers in Vietnam.

the further explanation of what superglue was supposed to be. I based my guess on the hard hat guy swinging from the I-beam commercial. bolshiabarmalay

Jäger is a big one here.

Jägermeister used to be a cough/cold medicine to help keep mountain climbers in the alps warm. To be fair, it still can keep you warm and it still tastes like cough/cold medicine. jimmyneyugn

Jäger is a big one here. It is actually a kräuterlikör, a kind of bitter alcoholic drink used to aid digestion in small quantities, and used to be seen as a drink for old people until a huge remarketing campaign.

Now it's what college students get wasted off of. Vinniam

Pop. Pop. Pop.

Bubble wrap. It was made to be a textured wallpaper. rosiix

That might be the stupidest invention ever, so after it's been up 6 months and you've bumped against it enough to pop most of them (or the kids got ahold of them) it now just looks like garbage. Nurum