A young saithe or coalfish (Pollachius virens), especially one in its second year.
Late 17th century. Origin uncertain. Perhaps from a transferred use of the unattested Norn cognate of Old Icelandic piltr (Icelandic piltur), Faroese piltur, Norwegian pilt, Old Swedish pilter (Swedish pilt), Danish pilt, all in sense ‘boy, lad’ (further etymology uncertain) + -ock. Or perhaps from Scots piltie penis (although this is first attested later), with suffix substitution; with the transferred use compare Orkney Scots pill young ling fish, apparently a transferred use of pill penis.