Backsplit 

The art of doing a backflip and ending doing the splits

We thought it was amazing at the start but nobody expected her to go into a backsplit.

Backsplit by Salanzarx November 18, 2018

Backlit

Ejaculating an unfathomable amount of cum on a human males back, typically after engaging in homoerotic activities with a male (or trans male).
The word is stemmed from the phrase "light up my back."

"Dude, how was Thailand? Did you get any of their cheap sex massages?"
"Naw dude, I paid them extra to backlit me! It was awesome!"

Backlit by Svensk och Indisk Pojke September 26, 2019

backsplash 

the water that sprays upwards from a toilet when you shit too hard

I knew that guy shits bricks...when he came out of the toilet he got backsplashed so bad he looked like he had a shower

backsplash by krak ho August 18, 2003

backsliding 

To go back to talking to, dating, or having sex with a previous ex. The backslider tends to be the person who broke it off, but not always. Friends must watch out for their friends who may be backsliding especially with those who have had a recent break up and/or if they don't think the two people should be together.

Everyone worries Rhianna is backsliding into an abusive relationship with Chris Brown.

backsliding by Catfish24 December 16, 2012

backspiration 

Sweat from the back region, particularly the small of the back, that turns your shirt game upside down. Grey t-shirts are particular harbingers of backspiration. The hotter and more humid it is, the bigger the backspiration stain. Sigh.

“Oh man, sometimes I hate walking around the city in the summer. It’s like backspiration central.”

backspiration by atomandeve December 11, 2014

backslide

verb: to have sex with someone you were previously dating or hooking up with. Specifically after a falling out or bad breakup and tends to make things more complicated.

"I knew I was going to backslide with Ben last night as soon as he walked into the bar. My friends are going to kill me"

backslide by citypretty21 March 1, 2011

Backspiration 

Backspiration is when someone sweats profusely from his/her back causing a big wet mark. Backspiration is an undesirable occurrence unless actively trying to ward off members of the opposite sex.

Normally I’m super embarrassed by my backspiration, but it sure does help keep Joe, that creep from accounting, at bay.

Backspiration by SupernaturalGoldfish December 10, 2014