This term is usually used after saying a non-intended humorous quote or phrase.

This term can also be used to put a serious twist on your joke, and frequently intensifies the situation.

It is also used to make a joke more humorous, this is called backing-up(usually used after your not-so-funny-joke)
#1:
Person 1: Yeh i got a vest the other day!
Group: HaHahahAhAh
Person 1: but seriously

#2:
Person 1: I've got a vest....jokes....but seriously
Group: HaHahahAhAh

#3:
Person 1: Vest are only for 'homos'
Group: *cough*
Person 1: but seriously!
Group: HaHahahAhAh
Used to back up a statement that is strongly true.
Person A: Dude, the performance of Berlioz's "Symphonie Fantastique" was the best thing ever!!

Person B: For seriously!
by Sho Fo-Sho March 10, 2009
"no, like, seriously we did!"
"don't go, seriously."
"seriously, you don't even know."
by BaBiedoll November 27, 2003
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
or
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)
by Kateus December 30, 2005
Disbelief in sarcastic form.
(As commonly used on the show Grey's Anatomy)
Meredith: You didn't sign the divorce papers. Fine. I get it. End of discussion.
Derek: Oh... I usually just say "Meredith" and then you yell at me. I haven’t thought past that point. I actually didn’t have anything planned. Meredith hits him repeatedly with her purse
Derek: Listen! Hey stop it! Ow!
Meredith: Seriously? Seriously?!
by Laurdev September 16, 2007
asking a question, slang word for emotion thats serious
jason: "she broke up with me and she wasn't even kidding, she was seriousing"
alex: "dude are you seriousing me? you guys went out for 3 years!"
by aakasd420 October 21, 2010
When she spoke to the crowd, her tone was one of seriousness.

His face showed intensity and seriousness as he strove to define "seriousness."
by Bill Foecke October 10, 2003