At a pinch; if necessary; in a pinch (archaic or dialectal form).
From Middle English 'a pinch' or 'at a pinch,' where 'a' is a preposition and 'pinch' refers to a critical situation, contracted into one word.
This word captures an old English phrase that's nearly extinct—we'd say 'if necessary' today, but apinch survives in some regional dialects as a linguistic fossil from centuries past.
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