Like a Mother Fucker. Also spelt L.A.M.F. and always capitalized.
Although two current UD definitons have it as 'punk' origin and used in the 70s, it was widely used as early as the 50s in Brooklyn and other areas of NYC and possibly beyond.
Sometimes it accompanied tags of gangs, but mostly it was used as a postscript with tags of gang members or wannabe gang members.
It was also frequently preceeded (separated by a space) by D.T.K. = Down To Kill
and followed less often by TTH = To The Heart