Meaning of “IsshouWaratteru(一生笑ってる)”

“IsshouWaratteru(一生笑ってる)” means laughing so hard it feels like it couls go on forever, kind of…
“Isshou(一生)” means a lifetime, and “Waratteru(笑ってる)” means laughing. This may be unnatural as a sentence, but I often see and here it used as slang. It’s impossible to laugh for a lifetime, of course, but it means something is so funny that it feels like you could.

Example Conversation

A: Look at this, it’s hilarious.
B: What is this? Oh wow, hahahahahaha
A: Right? I’m IsshouWaratteru(一生笑ってる) with this!

A: これ見てよ、うけるんだけど
B: なにこれ、やばいね、うける
A: でしょ、もうこれ見て一生笑ってる