Blocking or preventing someone from doing something, or excluding/not counting (as in 'barring any accidents').
From 'bar' which comes from Latin 'barra' (rod or barrier). The original sense was physical—putting a bar across a door—then extended metaphorically to stopping anything.
In medieval times, a 'barred' gate was literally blocked with a wooden bar, and 'barring' someone became synonymous with refusing entry; the medieval vocabulary of physical blocking shaped how we talk about all kinds of prevention!
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