Its a short term for Yea Fool, that has other variations, but this is the most common
person 1: wtf ur crazy
person 2: yafl
Y.A.F.L. - Yet Another Fewkin' Login - a phrase describing irritation at having to create another login for some online service, such as a forum or corporate website.
Grrr.. I need YAFL to access your New York Times article.