Well strictly speaking there isn't a 'singular'. The '-ism' derives (action) nouns from verbal stems. It's derived in this case from some chap called Onan who.. "...was the second son of Judah. After his older brother Er died, Onan was required by the tradition of levirate marriage to marry Er's widow Tamar. According to Genesis 38:7-10, when he had sexual intercourse with Tamar he "spilt his seed upon the ground" because the resulting child would be considered his late brother's, not his. In response to the transgression of disobedience, God killed Onan" But that seems more like Coitus Interruptus than well, you know... And actually, come to think of it, a sound thrashing was always considered a good solution to inappropriate hand shandy in the schools I went to - but capital punishment seems a bit strong.