Etymology
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payment (n.)

late 14c., paiement, "action of paying, repayment of a debt; amount due as a payment," from Old French paiement (13c.), from paiier (see pay (v.)). Meaning "thing or sum of money given in discharge of a debt" is from mid-15c.

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investment (n.)
1590s, "act of putting on vestments" (a sense now found in investiture); later "act of being invested with an office, right, endowment, etc." (1640s); and "surrounding and besieging" of a military target (1811); from invest + -ment.

Commercial sense of "an investing of money or capital" is from 1610s, originally in reference to the East India Company; general use is from 1740 in the sense of "conversion of money to property in hopes of profit," and by 1837 in the sense "amount of money invested." For evolution of the commercial senses, see invest.
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pennyworth (n.)

"goods costing a penny, as much as can be bought for a penny," Middle English peni-worth, from Old English peningwurð; see penny + worth (adj.). Figurative of "small amount" from mid-14c. Also generally, "value for the money given" (mid-14c.).

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earnings (n.)
amount of money one makes (from labor or investment), 1732, from plural of verbal noun earning, from earn (v.). Old English had earnung in sense "fact of deserving; what one deserves; merit, reward, consideration, pay," but the modern word seems to be a new formation.
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tantamount (adj.)
1640s, from verbal phrase tant amount "be equivalent" (1620s), from Anglo-French tant amunter "amount to as much" (late 13c.), from Old French tant "as much" (11c., from Latin tantus, from tam "so;" see tandem) + amonter "amount to, go up" (see amount (v.)).
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sum (n.)
c. 1300, summe, "quantity or amount of money," from Anglo-French and Old French summe, somme "amount, total; collection; essential point; summing up, conclusion" (13c., Modern French somme), from Latin summa "the top, summit; chief place, highest rank; main thing, chief point, essence, gist; an amount (of money)," noun use (via phrases such as summa pars, summa res) of fem. of summus "highest, uppermost," from PIE *sup-mos-, suffixed form of root *uper "over."

The sense development from "highest" to "total number, the whole" probably is via the Roman custom of adding up a stack of figures from the bottom and writing the sum at the top, rather than at the bottom as now (compare the bottom line).

General sense of "numerical quantity" of anything, "a total number" is from late 14c. Meaning "essence of a writing or speech" also is attested from mid-14c. Meaning "aggregate of two or more numbers" is from early 15c.; sense of "arithmetical problem to be solved" is from 1803. Sum-total is attested from late 14c., from Medieval Latin summa totalis.
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whit (n.)
"smallest particle," 1520s, from na whit "no amount" (c. 1200), from Old English nan wiht, from wiht "amount," originally "person, human being" (see wight).
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half-life (n.)

also half life, 1864, "unsatisfactory way of living," from half + life; the sense in physics, "amount of time it takes half a given amount of radioactivity to decay" is first attested 1907.

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convertible (adj.)

late 14c., "interchangeable," from Old French convertible "interchangeable" (13c.) and directly from Late Latin convertibilis "changeable," from Latin convertere "to turn around; transform," from assimilated form of com "with, together" (see con-) + vertere "to turn" (from PIE root *wer- (2) "to turn, bend").

Meaning "capable of being changed in form, substance, or condition" is from 1530s. Of paper money, etc., "capable of being converted into gold of a similar amount," from 1834. The noun is recorded from 1610s; meaning "automobile with a fold-down top" is from 1916. Related: Convertibility.

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moneyed (adj.)

"wealthy, affluent, having money," mid-15c., from past participle of Middle English verb moneien "to supply with money" (see money (n.)).

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