expression, godan verb (-ku)
expose one's genitals; show one's privates
Vulgar expression for deliberately exposing one's genitals, typically used in contexts of indecent exposure or lewd acts.
公園で男が突然前を開いた。
A man suddenly exposed himself in the park.
Compound of 前 (mae, 'front') + を (o, object marker) + 開く (hiraku, 'to open'), literally 'to open the front'. The meaning is idiomatic and refers to exposing the genital area.