1.
Cause to descend, lower, as in They let down the sails. [Mid-1100s]
2.
Also,
let up. Slacken, abate, as in Sales are letting down in this quarter, or They didn't let up in their efforts until the end. The first term dates from the mid-1800s, the variant from the
late 1700s.
3.
See let someone down. Also see let one's hair down.