caught with one's pants down

Etymology
Presumably engaged in some private or intimate activity. may refer to either trousers or underwear.

Adjective

 * 1)  Caught off guard, unprepared, or in an embarrassing situation.

Translations

 * Finnish: housut kintuissa
 * Russian: заста́ть враспло́х
 * Spanish: en paños menores, pillado con el culo al aire
 * Swedish:, med byxorna nere