Definition - a very stupid or foolish person
Pillock (which has also on occasion been spelled pilloch, pillok, and pillick) is one of the hundreds of euphemisms for the male sexual organ in the English language. The Oxford English Dictionary records the earliest known use of the word from the mid-16th century (a fruitful time for genitalia euphemisms), and for several hundred years this was apparently the main sense of the word. However, beginning in the late 20th century pillock took on another meaning, which is that of an idiot or fool of some sort. Both of these uses are almost entirely confined to British English, and the word has little currency in the United States.
There is a season-ticket holder at Southampton to whom every referee will always be a “daft pillock.”
—The Guardian (London, Eng.), 17 Jan. 1977