Weenus

This is slang for the loose skin on your elbow, when your arm is straight (when it’s flex, the skin is tighter around the bone).

Merrium-Webster did some research on the source.

Being a slang term most often used in social media, it is difficult to determine exactly when weenus was first used or how it developed. However, online evidence does suggest that it was probably coined in the late 20th century, and by the early 21st century, the word gained widespread circulation. Etymologically, it seems likely weenus is a play on the medical term penis and its slang variant wiener.

So, basically, it was made up.

They also mention the Friends episode where I thought it came from:

[…] a 1995 episode of the popular television comedy Friends during which the character Chandler Bing has to explain that the humorous-sounding acronym WENUS represents “Weekly Estimated Net Usage Systems.”

Here’s the clip.