After a sentence, either sarcastic or serious when people have started laughing, use 'No seriously' to either show
A) You're serious and want them to stop laughing because it's not funny
B) You're taking the piss out of them by not being serious
Amy "Hey Ashlee, do you want to come with me and my extended family to woop woop
for a month or two? It'll be super fun!"
Ashlee (laughs) "Yeah, a month or two in woop woop
with your extended family will be super fun!"
Amy (pissed off) "No, seriously. I think I'll go ask Sarah. (under her breath) Bitch."
Jim "Dude, last night I had a totally rippin' threesome with Carmen Electra and Jessica Simpson!"
Joe (laughs) "Like fuck you did"
Jim (laughs) "No, seriously. I jacked off alone to Judge Judy."
Joe (disgusted) "Oh, dude" (leaves)