Meaning:
Lit. “to go (move into) saliva”. This is a very colloquial and somewhat vulgar way of describing a passionate kiss during which two individuals explore each other’s oral cavities and exchange some body fluid in the process. The act quite often precedes an even more serious sexual activity and by using this expression you provoke a question about what happened next.
This seems to be a quite fresh addition to Polish vocabulary and I had not been aware of its existence until a colleague used it commenting on another married chap’s improper behaviour in a bar.