If you’re not from Britain, this term may not be all that familiar to you. Allow me to quote the synonyms given in Visual Thesaurus: dolt, stupid person, stupid, stupe, pudding head, pudden-head, poor fish, dullard. To these I think I could reasonably add a common term from Canada: dickhead.
I think that’s a viable synonym not just because of the resentfully abusive way in which the word pillock tends to be used – it carries implications of not just dullness but obnoxiousness too – but also because of its literal reference.
Oh, you don’t know what part of the body the pillock is? Would you care to make a guess? The Oxford English Dictionary reckons that it’s probably shortened from pillicock. The pill is Scots and northern English dialect, probably taken from Scandinavian influences (the Danes used to run that part of the country); it refers not to, say, a little blue pill such as Viagra but rather to that part of the body that the little blue pill is meant to affect. The rest of the word pillicock should be sufficiently obvious. As to the aphetic form pillock, I have to wonder whether it may not have been affected by bollock, a word usually seen in the plural (like its referent).
The word seems like what you get when you expect a pillow and get a rock. It has a taste of someone who gets in a mixed-up pickle, someone who brings ill luck. And at the same time the /p/ and /k/ with a liquid and /ɪ/ between have a bit of a taste of prick as well as bilk and a bit of an echo of kill.
Although the word has been in at least some versions of English since the 1500s, it hasn’t been very evident in print until the last third of the 20th century, when it started being used as a term of abuse. I can’t say whether there was one particular work that served as a primary vector for this, but the word evidently caught on like wildfire.
I like this definition of pillock from 1978 in Approach: The naval aviation safety review: “An idiot who, having gotten himself into the wrong lane, then expects everyone else to give way so that he can get to where only he knows he wants to go, with or without use of flashing indicators or consideration for traffic flow.” You can see where, with such images in mind, the word might have proven quite appealing for general use. And might seem a nearly exact synonym for dickhead.