Colloquial / dialectal reading. Survives in northern speech and in fixed words like 尿泡 and 尿脬 (bladder). For all standard medical and everyday vocabulary, use 尿 instead.
Inner 水 - pictograph of flowing water. With the body radical above, this is one of the most starkly literal compound ideographs in the script: a body releasing water = urine. No phonetic component; both pieces are pure meaning.
