A nursery euphemism for urine or urination.
Eric Partridge records this in his A Dictionary of Slang, 1937, as "Wee-wee, a urination; esp. do a wee-wee" and considers it to be late 19th century.
Reduplications that are formed from repeating words like this are symptomatic of the infantile way that youngsters used to be spoken to; for example, choo-choo, goody-goody etc. This fashion is changing rather and such terms are becoming less commonly used.
Wee, and Wee-wee, may well be variant of pee. That itself is a jokey euphemism and appears to derive from the 'P' in piss. Shakespeare seems to use it that way in Twelfth Night, 1602:
"By my life, this is my lady's hand: these be her very C's, her U's, and her T's, and thus makes she her great P's."
Never shy of recycling a good old joke, Talbot Rothwell, the screenwriter of most of the Carry On series of films, used this one again in Carry On Camping. A sign on a field gate says 'All asses must be shown'. Sid James approaches the gate:
Are you the owner of this site?
Where is he?
Gone for a P.
See other reduplicated phrases.
See other phrases and sayings from Shakespeare.