It is a phrase used mainly in Ireland, that means this particular woman cant keep a secret, especially when she is drunk.
Guy 1:Did you here what she said about Sean last night?
Guy 2: Sure you know that after a few beers she can't hold her piss.
Guy 2: Sure you know that after a few beers she can't hold her piss.
by GERDAN FANTANA May 15, 2011