'Willie' is quite acceptable IMO and I could probably supply you with a dozen or more less acceptable euphemisms too.
Our English kanguage is rich and colourful and should never be censored or limited to basic scientific terms. I love the way words evolve and some are quite amusing, especially to the innocent and unwary.
I'll never forget nearly inhaling my tea when my late (and naive) MiL, talking about some VIP, declared him to be a 'big knob' 
What colour shoes/boots to wear.
CONTINUATION of write a word beginning with the first letter of the name of the user above you



which reminds me of that wonderful Blue Peter clip at the door of some cathedral, admiring the medieval knocker. Much sniggering at that!