〜らんない means “can’t (stand to)…”. In fast speech the れ of られない turns into ん: 信じられない becomes 信じらんない, やってられない becomes やってらんない (“I can’t take this anymore”).
Is 〜らんない rude? Who can you say it to?
With friends: OK. With coworkers: careful. With a boss or teacher: avoid. Casual. At work, keep 〜られません.
How common is 〜らんない in real speech?
In 1,905 live comedy routines, 〜らんない appears in 39 lines from 29 comedians.