Green’s Dictionary of Slang

1. a sudden humiliation, esp. the inadvertent exposure of the buttocks or genitals.

1970

(con. 1950s) H. Junker ‘The Fifties’ in Eisen Age of Rock 2 (1970) 101: Certain college studs [...] displayed their naked asses to passersby, an act variously called [...] handing out the b.a., gotcha.

2. in fig. use, something that surprises, esp. with the potential for humiliation.

1999

Guardian Guide 5–12 June 57: This week’s gotcha’s include Jerry Springer’s porn-star romp.