godan verb (-tsu), intransitive verb
to have an erection; to become erect
Colloquial term for penile erection. Often used in casual or vulgar contexts. The kanji 勃 is specifically associated with erection, distinguishing it from 立つ (to stand).
朝起きたら勃ってた。
I woke up with an erection.
I get hard quickly watching that adult video.