(idiomatic) A disappointment or anticlimax.
Example: 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.
1990, WJA Payne, An Introduction to Animal Husbandry in the Tropics:
The majority of peasant farmers in the tropics allow the calf to suckle before milking in order to obtain a let-down of milk.
2004, The Womanly Art of Breastfeeding (7th edition), Penguin 2004, p. 54:
Occasionally a baby will be caught off-guard by mother's strong let-down and he will choke and sputter a bit.
2010, Jan Riordan & Karen Wamback, Breastfeeding and Human Lactation (4th edition), Jones & Bartlett 2010, p. 91:
Through oxytocin mediation, these afferent pathways become so well established that letdown can occur even when the mother merely thinks of her baby.