"I promised him I would meet him there, and I will not let him down."
2.VerbTo lengthen by undoing and resewing a hem.
3.VerbTo reduce one's level of effort.
4.VerbTo soften in tempering.
"to let down tools or cutlery"
5.VerbTo thin; to reduce the thickness or viscosity of.
"let the sauce down with a little pasta water if it starts to become too thick; if the soup is too thick, you can let it down with a little milk; let the sauce down with a dash of the pickling liquor; use some of the stock to let down the onion sauce"
6.VerbUsed other than figuratively or idiomatically: see let, down: especially, to allow to descend.