Make known, reveal, as in I thought it was a secretwho let it out? [First half of 1800s] Also see let the cat out of the bag.
Come to a close, end, as in What time does school let out? [Late 1800s]
Increase the size of a garment, as in May's coat needs to be let out across the shoulders. This usage refers to opening some of the seams. [Late 1700s]