flat as a tack

(as) flat as a tack

Demoralized or unenergetic. She's been flat as a tack ever since she heard that she didn't get into her top choice school.
See also: flat, tack

flat as a tack

in very low spirits or lacking in energy. informal
The idea underlying the expression is of a tack that has been hammered in so that none of it protrudes.
See also: flat, tack