Wiktionary
spend a penny
vb. (context British euphemistic idiomatic English) To use the toilet.
WordNet
spend a penny
v. eliminate urine; "Again, the cat had made on the expensive rug" [syn: make, urinate, piddle, puddle, micturate, piss, pee, pee-pee, make water, relieve oneself, take a leak, wee, wee-wee, pass water]