(1) adv - "seriously?", expressing doubt/surprise or asking for confirmation
(2) exclamation/statement issuing confirmation
(3) agreement
(1) yo girl is preggers with triplets?! for serious?!
(2) for serious, that chick wants my ass!
(3) "that teacher is dumb ass hell."
"for serious, man."
a combination of the phrases "for real" and "seriously", "for serious" can express agreement when used as an exclamation, it can also be phrased as a question.
Example 1:
person A: "Hey, I got a newcell phonetoday"
person B: "For serious?"
Example 2:
person A: "That girl is such a ditz"
person B: "for serious!"