let down verb 5/ˈlɛtdaʊn/ noun 3forms 3 verb5 •To disappoint; to betray or fail somebody.I promised him I would meet him there, and I will not let him down. •To reduce one's level of effort. •To lengthen by undoing and resewing a hem. •To 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 •To soften in tempering.to let down tools or cutlery let-down /ˈlɛtdaʊn/ noun3 •A disappointment or anticlimax.After seeing all the advertisements, the show itself was quite a let-down. •The neurohormonal release of milk in dairy cows or in breastfeeding human mothers.The majority of peasant farmers in the tropics allow the calf to suckle before milking in order to obtain a let-down of milk. — 1990 •The clearance of an aircraft to descend through clouds to clear air below, or the process of granting such a clearance.The flight received a radar letdown from air traffic control. formsletting down · lets down · let-downs