letdown

noun

1. a decrease in volume, force, energy, etc.: a letdown in sales; a general letdown of social barriers.
2. disillusionment, discouragement, or disappointment: The job was a letdown.
3. depression; deflation: He felt a terrible letdown at the end of the play.
4. the accelerated movement of milk into the mammary glands of lactating mammals upon stimulation, as by massage or suckling.
5. Aeronautics. the descent of an aircraft from a higher to a lower altitude preparatory to making an approach and landing or to making a target run or the like.

n.also let-down, “disappointment,” 1768, from let (v.) + down (adv.). The verbal phrase is from mid-12c. in a literal sense; figuratively by 1795.