expression, ichidan verb
Vulgar or crude expression for urinating, used mainly in rough or informal male speech. 小便 is a coarse word for urine, and 垂れる adds a sense of letting it drip or flow.
酔っ払いが道端で小便を垂れていた。
A drunk was pissing on the side of the road.
小便 (shōben, 'urine') + を + 垂れる (tareru, 'to let hang down, to drip'). The reading しょんべん is a colloquial or dialectal variant of しょうべん.