From Ovid's Narcissus (which is where narcissism is derived) As students in England, we were taught this was the origin of the derogatory term "cissy" (a man who loves himself or loves one who looks like himself) I have no proof - only my memory.
I parenthesized "prevaricatin'" because I anticipated that a FReeper such as yourself would pin me to the wall.
And you did;)
sp
Well, it does sound a bit fanciful to me, though the fact that "cissy" is the British spelling gives it slightly more credence there.
Still, "sister" seems the most likely from a semantic point of view. Sister - woman - weak - cissy
Narcissus was defined by his egocentrism rather than cowardliness or weakness.