In a manner that deviates from what is normal or expected.
From Latin 'ab-' (away from) plus 'norma' (carpenter's square, rule). The concept moved from physical measuring tools to social and statistical standards, with 'abnormal' describing deviation from established patterns.
The carpenter's square origin reveals how 'normal' originally meant conforming to a physical measuring standard! Our modern statistical concept of normal distributions and abnormal variations still carries this ancient idea of measuring against a reliable reference point.
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