The debate on the actual meaning of the word is between that Weenis is referring to the skin on the outside and the pit of the elbow, and it being a silly portmanteau of “wiener” and “penis”.
However, the majority of people claim that originally “Weenis” is the skin on your elbow, and only children and adolescents use it as a slang referring to a “penis”.
The term also has its alternative, including “wussy” which is a combination of “wimp” and “pussy”.
“Weenis” and “Wagina” may also be used as insults.
While the exact origin of the term “Weenis” and its alternatives are unknown, they definitely started to be used as a slang amongst children and young adults around the 1990s.
Also, the term circulating around the internet started in the early 2000s, and since then it still has been around as a known slang.
The term is not always used to describe that part of the elbow, not even in the mainstream media.
For instance, in a TV sketch comedy Mr. Show with Bob and David, a 1998 episode was titled “Show Me Your Weenis!”.
Influenced by an episode of the famous sitcom Friends, “weenis” can be sometimes spelled as “wenus”.
Likely, because of a joke on a 1996 episode of Friends involving the acronym “wenus”, Weekly Estimated Net Usage Systems.