Avoir le cul entre deux chaises

Literally: To have one's butt between two chairs

Example: “Entre ses deux offres d'emploi, il a le cul entre deux chaises.” — Between his two job offers, he's torn and can't decide.

Origin: The Académie française's own 1694 dictionary already has the idea, if not the modern wording: of a man who had staked his hopes on two different things and secured neither, it says he finds himself entre deux selles le cul à terre — between two stools, backside on the ground. The image needing no explanation: try to sit on two chairs set apart and you succeed at neither, ending up worse off than if you had picked one. The current phrasing is attested by the end of the 18th century, in a dictionary of Provençal and the Comtat Venaissin, and cul (backside) eventually displaced the more decorous selles (stools) as the wording spread through the 19th and 20th centuries.