'Wankshaft' is an insult, calling someone a 'wanker' and a 'dick', in one word.
The phrase coming from the idea that the shaft, the length of the male member, is solely being employed for wanking and not for procreation.
The sin of Onan, or onanism, is not now inherent in its use as it is rarely used to describe actual masterbatory practices, but more usually the extrapolation of the term wank as a noun.
Whilst the term wanker is usually used in either vitriol or as a term of endearment, the word wankshaft always implies digust and disdain.
"Wadda fucking wankshaft"
"How did he even get a job over here?"
"Coz he was so useful uncovering War Crimes as editor of the Mirror they thought they'd put him to good use working out if The Hoff = "American's Got Talent" - its like algebra or summit"