(informal) if, in case

(informal) for once, for a change

there was once…; once upon a time, there was… (traditional beginning of children’s stories, especially fairy tales)

(idiomatic) once and for all (finally, permanently, conclusively)

doubly, so as to make sure, so that there be absolutely no doubt / absolutely! with pleasure! yes please!

Alternative form of une fois pour toutes

(figuratively) to think twice

once in a blue moon (very rarely or never)

you only go around once, you only live once, YOLO

doing it once is all right, but let it not turn into a habit; it's ok this time, but let's not make a habit of it