Leashes are commonly associated with restraining or keeping something under control. So, antonyms for "un-leashes" would include words such as restrain, hold back, limit, control, or contain. These words imply that some form of restriction or confinement is being imposed to prevent the subject from moving freely or expressing itself fully. Conversely, "un-leashing" suggests the opposite, that something is being released or let loose. Synonyms for "un-leash" include words such as unleash, free, liberate, discharge, or erupt. These words suggest that something is being released from bondage or allowed to express itself without restriction, often resulting in a sudden and explosive burst of energy or emotion.