Think I've looked this one up before and all I can find is:

Early 19th century - the battle of the sexes was raging even then! ... Original words were probably 'snips of snails' i.e. 'little bits of'.

I shall be doing my research between now and then .....

My simple brain seems to be telling me that a childish drawing, comprising snails and/or a puppy dog's tail, could well appear as an abstract picture of male genitalia. ooooh YUK

Not sure where 'the snip' comes into the equation. (Isn't that what we now call a vasectomy? YIKES)

But HEY! Maybe I over think these things :P

... back to the 'drawing board' :D