This textual acronym can either mean "fuck you/yourself" or "fuck yeah."

lil mad is like big mad but the opposite, you mad but not mad to the extent of pullin somthin

Short for fuck yeah.

"FY" is the abbreviation word for "For you ;)". It is used to show an unconditional and sincere appreciation for what someone just said to you by returning the same compliment. It always follows the word "Thank you for you".
Fun fact: "FY" is used a lot in Azerbaijan.

internet acronym for "Fuck Yeah!"

(people often mistaken the "!" as an inverted "i" expecting it to be FYI or "for your information")

Short for "f**k you"

Used as a prefix on maps in Counter-Strike that means "fight yard." The most fun maps of them all.