Verlan flips a word's syllables — the name itself is l'envers (the reverse) turned around: l'en-vers → vers-l'en → verlan. A handful of verlan words are now part of everyday spoken French, used well beyond the young:

Verlan	From	Meaning
relou	lourd	annoying, a pain
chelou	louche	dodgy, weird
ouf	fou	crazy, amazing
vénère	énervé	wound up, angry
meuf	femme	woman, girlfriend
laisse béton	laisse tomber	forget it (dated)

C'est ouf ! can mean it's crazy or it's amazing — the tone decides. All of these are familier: fine with friends, never in an email or an interview.