Entering someone else's land or property without permission, which is against the law in most places.
From Old French 'trespasser,' meaning 'to transgress' or 'to go beyond.' The prefix 'tres-' means 'across,' so trespassing originally meant crossing a boundary you shouldn't cross.
The word 'trespassing' is tied to the entire concept of private property—without it, we wouldn't have a legal way to say 'this land is mine and you can't be here!' It's one of those words that literally created social order.
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