Coloquialism for male genitalia (Penis), predominantly used in Scotland.
Recently used as a double-entendre in a number of songs by Real Radio breakfast presenters Robin Galloway and Kat for example "I love ma boaby (on a Saturday night)" where Cecil's boaby is explained to be a small mouse.
Used on a number of wind-ups on the same show... e.g. I've stuck my boaby up my exhaust
From the song
"I love ma boaby, Kat loves the boaby"