S.F.L.A.N is an abbreviation of the term "Sucka for love ass nigga", meaning a man who will act a complete fool and do anything for a particular woman. It was popularized infamous Wu-Tang Clan member , Ghostface Killah.
Yo you spent all your rent money on a pair of new boots for that chick? You's a SFLAN for real. Suka for love ass nigga.