the.com/letdown

means A disappointment, especially the flat, deflated feeling that follows when something fails to live up to what was expected or promised.

from A straightforward compound of 'let' and 'down' — the literal image is of something being lowered or allowed to drop, like slackening a rope. The sense shifted in the early 20th century from physical slackening to emotional deflation: a letdown is when expectation, held taut, goes slack. Aviators and engineers also used 'letdown' for a controlled descent, which only reinforces the core feeling — the slow, sinking comedown from altitude.