Etymology
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makeweight (n.)

also make-weight, 1690s, "small quantity of something added to make the total reach a certain weight," from make (v.) + weight. Meaning "thing or person of little account made use of" is from 1776.

MAKE WEIGHT. A small candle: a term applied to a little slender man. [Grose, "Dictionary of the Vulgar Tongue," London, 1785]

For the formation, compare makeshift, also make-sport (1610s), makegame (1762) "a laughing stock, a butt for jokes;" makebate "one who excites contentions and quarrels" (1520s); makepeace "a peace-maker, one who reconciles persons at variance" (early 13c. as a surname).

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makeshift 

also make-shift, 1560s, as a noun, "shifty person, rogue" (a sense now obsolete; for the formation, compare makeweight), from make (v.) + shift (n.). As an adjective, 1680s, "of the nature of a temporary expedient," which led to the noun sense of "that with which one meets a present need or turn, a temporary substitute" (by 1802).

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mantissa (n.)

"decimal part of a logarithm," 1865, from Latin mantisa "a worthless addition, makeweight," perhaps a Gaulish word introduced into Latin via Etruscan (compare Old Irish meit, Welsh maint "size"). So called as being "additional" to the characteristic or integral part. The Latin word was used in 17c. English in the sense of "an addition of small importance to a literary work, etc."

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