Merely means 'only' or 'nothing more than', often used to make something sound smaller or less important. It downplays what follows.
From Middle English “merely” meaning 'completely, purely', from “mere” 'pure, unmixed, absolute', from Old English “mǣre” 'pure, unmixed'. Over time, the meaning shifted from 'purely' to 'only, just', often with a minimizing tone.
Merely used to mean 'purely', but English twisted it into 'only'—almost the opposite mood. When you say 'I’m merely asking', you’re doing a tiny piece of spin, shrinking your action so it seems harmless.
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