Behind means at the back of something or hidden by it. It can also mean later than or slower than others in time or progress.
“Behind” comes from Old English “behindan,” combining “be-” (by) and “hindan” (from behind). It has always carried the sense of something at the back or in the rear.
We use “behind” for space, time, and success: you can stand behind a tree, fall behind schedule, or feel behind in class. Your brain reuses the same “back of the line” picture for all three without even noticing.
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