Definitions
1.VerbTo allow to descend."They let down the rope and I fastened it to the basket."
2.VerbTo disappoint; to betray or fail somebody."I promised him I would meet him there, and I will not let him down."
3.VerbTo lengthen by undoing and resewing a hem.
4.VerbTo reduce one's level of effort.
5.VerbTo soften in tempering."to let down tools or cutlery"
6.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"
7.VerbUsed other than figuratively or idiomatically: see let, down.