Si ma tante en avait, ce serait mon oncle

Literally: If my aunt had some, she'd be my uncle

Example: “« Si on avait pris l'autre route, on serait arrivés à l'heure. — Oui, et si ma tante en avait, ce serait mon oncle. »” — "If we'd taken the other road, we'd have been on time." "Sure, and if ifs and buts were candy and nuts, we'd all have a merry Christmas."

English equivalent: If ifs and buts were candy and nuts

Origin: A bawdy piece of folk logic: the unspoken "en" refers to male genitals, so the aunt would indeed be an uncle. It is used to shoot down arguments built on "si…" — the same job done by the milder "avec des si, on mettrait Paris en bouteille." The joke has close counterparts in other languages, including the English "if my aunt had balls, she'd be my uncle" and the Italian "se mia nonna avesse le ruote, sarebbe una carriola." Its date and first appearance are not documented.