(OED) says the word is of "Origin unknown" but adds: "Ogilvie compares 'Gypsy and Hindi loke a man.'" The OED's first cited use is in 1861. Some believe it derives from the Celtic word ploc, a large, bull-headed person. The word first appears in early 19th century England possibly, according to Michael Quinion, as a variation of the slang term gloak, which itself was a variation of an older slang term buzzgloak, meaning pickpocket.

History