have one's heart in one's boots

A kicking, as with a booted foot.

(Ireland) To give one a sudden fright.

In baseball slang, to boot one means to make a fielding error — specifically to fumble or misplay a ground ball, as if you'd clumsily kicked it rather than fielded it cleanly. The metaphor of the boot implies ungainly, clumsy failure. More broadly in casual American English it extends to mean making any significant blunder. Sports journalists and commentators use it freely, and it carries some general informal currency beyond baseball.

(originally, US, informal, frequently in the phrase 'get cold feet') A feeling of nervousness or anxiety causing one to reconsider a decision about an upcoming event.