The front (formerly also back) flap of the trousers or underpants; the fly.
Origin
Late 16th century; earliest use found in Robert Lindsay (c1532–c1586), historian. Probably from bag + lap, with assimilation of the final consonant of the first element.
Definition of ballop in US English:
ballop
(also ballup)
nounˈbaləp
Scottish
The front (formerly also back) flap of the trousers or underpants; the fly.
Origin
Late 16th century; earliest use found in Robert Lindsay (c1532–c1586), historian. Probably from bag + lap, with assimilation of the final consonant of the first element.