(informal, litotes) not to feel too at ease, i.e. to be scared out of one's wits

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

(dated) to threaten, to guarantee that there will be hell to pay, that there will be severe retribution, that there will be trouble

(transitive) not to let out of one's sight, not to take one's eyes off, to keep an eye on

(vulgar) to do fuck all, not to do a damn thing, to do nothing

(informal) God forbid!

(informal) that'll be the day! don't hold your breath!

(informal, of people) each is as bad as the other, they/you are all uniformly bad, they/you are all a worthless, incorrigible bunch

(informal) and that's not the half of it!

to be something to which one wouldn't say no, to be something that one wouldn't mind, to be most welcome