Noun. From Japanese
“A man who is willing but is under-endowed”
I had lots of fun finding a suitable picture for this, and I’m pretty sure these two could be classed as ikibari (although, I can’t help but look at the one on the left and think to myself… Marge Simpson?)
Its use is as an insult and while there are very similar connotations that can be given in English and other languages, none of them seem to describe the sense of eagerness or willingness that an ikibari would have.
Perhaps the closest recent slang word I can think of, and the search term that lead me to the above photo, is douchebag, but that seems to incorporate a whole other mess of things (mess being the keyword here).