fwiw all mine have "meaning" for me, but yeah, they need not.
as to the one you're interested in, it might be something like don't bite the hand that feeds...., because superiors will punish you for transgressions. BUT the knife through it implies you SHOULD transgress: F*** the man.
you also figure that hands represent manual labor: digging in the earth and shit. and so they've got like muscle memory: musicians practice in order to get their hands automatic using muscle memory.
that's what comes to mind anyway.