Short for residue or resident, commonly used in technical contexts to refer to remaining material or a person in residence.
Abbreviated form of 'residue' (from Latin residuum, 'that which remains') or 'resident' (from Latin residens, 'remaining, dwelling'). The clipped form emerged in the 20th century in specialized professional jargon.
In the oil refining industry, 'resid' specifically refers to the heavy, viscous material left over after lighter fractions have been distilled away—it's essentially the 'bottom of the barrel' that gets processed into asphalt or heavy fuel oil. Medical residents also use this term as slang for themselves!
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