you must leave at once : IMMEDIATELY, right away, right now, this moment/instant/second/minute, now, straight away, instantly, directly, forthwith, promptly, without delay/hesitation, without further ado; quickly, as fast as possible, as soon as possible, a.s.a.p., speedily; informal like a shot, in/like a flash, before you can say Jack Robinson.
all the guests arrived at once : AT THE SAME TIME, at one and the same time, (all) together, simultaneously; as a group, in unison, in concert, in chorus.
2192; headword