/ ənˈles; NAmE / conjunction
1.
used to say that sth can only happen or be true in a particular situation :
You won't get paid for time off unless you have a doctor's note.
I won't tell them—not unless you say I can.
Unless I'm mistaken, she was back at work yesterday.
He hasn't got any hobbies—unless you call watching TV a hobby.
2.
used to give the only situation in which sth will not happen or be true :
I sleep with the window open unless it's really cold.
Unless something unexpected happens, I'll see you tomorrow.
Have a cup of tea—unless you'd prefer a cold drink?
HELP NOTE : Unless is used to talk about a situation that could happen, or something that could be true, in the future. If you know that something has not happened or that sth is not true, use if ... not :
If you weren't always in such a hurry (= but you are) , your work would be much better.
Your work would be much better unless you were always in such a hurry.
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WORD ORIGIN
late Middle English : from on or in (assimilated through lack of stress to un- ) + less .