'weenis' is the extra skin on your elbow
'butt' is your gluteus maximus, or for you idiots it's the ass therefore when combined it is the extra skin on someones glutues maximus.
usually used when insulting someone or just complimenting them.
"you're a fuckin buttweenis!" or "wutssup buttweenis?"
Someone who is born between Gen X and Millennials and often identifies with both or neither generation. Loosely defined as those born between 1978 and 1982.