解手 is a separable verb (离合词) that primarily means 'to go to the toilet' — it's a euphemism with a historical origin: during the Ming dynasty, immigrants were tied by their hands, and when they needed to relieve themselves, they would ask to have their hands untied, hence the phrase. The literal meaning 'to untie one's hands' still exists but is very rare in modern usage. As a separable verb, you can insert a measure word or duration between the syllables, e.g. 解了一次手 ('went to the toilet once').