Looking Slack-Jawed At My Monitor. Incredulity at what one is seeing or reading on the Internet.
As a statement:
I was on 4chan and I was all LSJAMM.
As a response:
Biff: I just had sex with my mother.
About someone else:
That Russian kid seeing porn for the first time was totally LSJAMM.
Guy #1: Dude, check the hottie.
Guy #2: El-es-jam for sure