Either means, wow thats pretty cool. or damn good job!! Or sarcastic like, wow ur a fag i can't believe you did/said that...
1: im going to go buy my costume for the star wars premier.
1: dude i just got laid!
Means acknowledgement to what is sometimes, but not neccessarily always, something dumb/stupid.
(In between 'nice' (said quickly), meaning good, and 'niiiiiice' (stretched out), meaning something really good.)
In 'niice' the double 'ii' is stressed.
Person1: OMG, I just spilled water on my keyboard!!
Person1: Dude! I just beat this one guy at pool and I've never played before!!
way nicer that nice. it's niice, pronounced by elongating the "i" sound so that it almost sounds like nahhs.
Person 1: Dude I'm so highhhhh.....
Person 2: Niice!
Person 1: Bro I just found 20 bucks on the ground.
Person 2: That's fucking niice!