godan verb (-tsu), intransitive verb
become erect; tower like an erection
Vulgar pun on そそり立つ (to tower, rise up), using 勃つ (to get an erection). Used for penises, nipples, or things that stand up suggestively.
See also: そそり立つ (そそりたつ)
彼の股間がそそり勃っているのが見えた。
I could see his crotch was erect.
その塔はまるでそそり勃つかのようにそびえていた。
The tower stood tall as if it were an erection.