During the period l966 to 1967 a group of anarchists residing in a squat on Manhattans Lower East Side called themselves "The Motherfuckers".
Living communally, they were one of the far more openly militant groups of their time, relied upon by the SDS and other (more secretive) radical organisations to provide muscle during street riots and subsequent confrontations with the Police.
The SDS at that time were busily up to their own variety of nasty tricks, for example bombing police stations and police cars, hence their interest in - and some even charge use of - the Motherfuckers to provide distractions for their more nefarious activities.
Of course Cops pride themselves on their "hippness", and even now tend to refer to gang members by their group association so any assumption of anonymity on the part of gang members is immediately be shown to be false.
Hence the expression "UP AGAINST THE WALL MOTHER FUCKER!"
In later the sixties and early seventies The MC5 (Motor City 5 from Detroit) had a song of the same name.
Finally, poet Le Roi Jones, later known Amiri Baraka - originally a Greenwich Village beat poet, but later a black nationalist - used the same line in many works.
In later years he openly followed and endorsed Malcolm X, so the Village Voice and other liberal publications of that time didn't appreciate him so much anymore.
His muslim name is (was? I'll check and update this node as I can research) Baraka.