Partirse el pecho de risa

Partirse el pecho de risa
 

when sth is really funny and we start laughing and can´t stop, we say partirse el pecho?
ENGLISH?

To split your sides laughing.

Also:
To bust a gut.

Oh. I thought that was to make a big effort, conceptually related to overdoing it and getting a hernia.

I have heard split a gut for something truly hilarious.

You're correct. The phrase has both meanings.
I've heard Poli's contribution before, and a couple of others.
'To laugh one's head off' (and other body parts) is another very common way to say the same thing.