if you look at everything like that then you'll always wonder...like..why is a chair called a chair..or a computer called a computer...
most usually it's from it's latin or greek name or something of that sort...
see..according to wikipedia (wikipedia answers many questions...yes i looked it up)
The word masturbation is believed to derive from either the Greek word mezea (μεζεα, "penises") or the Latin manus ("hand") and the Latin turbare ("to disturb").[6] A competing etymology based on the Latin manu stuprare ("to defile with the hand") is said by the Oxford English Dictionary to be an "old conjecture