'Cock' is an abbreviation of 'Pillicock',
which is also abbreviated to 'Pillock'.
Manley's etymology fact of the day.
(
LordManley twitter.com/LordManley,
Tue 16 Sep 2008, 9:08,
archived)
but cock is sqwearing
and pillock isn't
(according to my mum)
(
Joe Scaramanga with a G-double-O-D vibration,
Tue 16 Sep 2008, 9:23,
archived)