1Urine, or the act of urinating. A common everyday word, but slightly more blunt than 'oshikko' and often avoided in polite settings.
子供が「小便がしたい」と言い出した。
The child suddenly said, 'I need to pee.'
運転中、彼は小便を我慢していた。
While driving, he was holding in his need to urinate.
健康診断で小便の検査をした。
I had a urine test done at the health checkup.
犬が電柱に小便をかけていた。
The dog was peeing on the utility pole.
2Slang for backing out of a promise or contract without warning; casual usage common in gambling contexts.
土壇場で小便するような人とは二度と組みたくない。
I never want to team up again with someone who bails out at the last minute.