Etymology
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expense (v.)

"offset (an expenditure) against an income," 1909, from expense (n.). Related: Expensed; expensing.

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expense (n.)

also formerly expence, late 14c., "action of spending or giving away, a laying out or expending," also "funds provided for expenses, expense money; damage or loss from any cause," from Anglo-French expense, Old French espense "money provided for expenses," from Late Latin expensa "disbursement, outlay, expense," noun use of neuter plural past participle of Latin expendere "weigh out money, pay down," from ex "out, out of" (see ex-) + pendere "to hang, cause to hang; weigh; pay" (from PIE root *(s)pen- "to draw, stretch, spin"). For the financial sense of the Latin verb, see pound (n.1). 

Latin spensa also yielded Medieval Latin spe(n)sa, the sense of which specialized to "outlay for provisions," then "provisions, food" before it was borrowed into Old High German as spisa and became the root of German Speise "food," now mostly meaning prepared food, and speisen "to eat." Expense account is from 1872.

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DOS 

"computer operating system using a disk storage device," 1967, acronym of disk operating system.

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undercover (adj.)

1854, "sheltered," from under + cover (n.). Sense of "operating secretly" attested from 1920.

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modus operandi (n.)

"way of doing or accomplishing," 1650s, Latin, literally "mode of operating" (see modus). Abbreviation m.o. is attested from 1955.

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boot (v.2)

1975, transitive, "start up (a computer) by causing an operating system to load in the memory," from bootstrap (v.), a 1958 derived verb from bootstrap (n.) in the sense of "fixed sequence of instructions to load the operating system of a computer" (1953).

This is from the notion of the first-loaded program pulling itself (and the rest) up by the bootstrap. The intransitive use, of a computer operating system, is from 1983. Related: Booted; booting.

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sumptuary (adj.)

"pertaining to expense," c. 1600, from Latin sumptuarius "relating to expenses," from sumptus "expense, cost," from sumere "to borrow, buy, spend, eat, drink, consume, employ, take, take up," contraction of *sub-emere, from sub "under" (see sub-) + emere "to take, buy" (from PIE root *em- "to take, distribute").

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expenses (n.)

"charges incurred in the discharge of duty," late 14c. See expense (n.).

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Windows (n.)

the computer operating system by Microsoft was introduced in 1985 and modified thereafter; it was predominant by c. 1995.

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coplanar (adj.)

also co-planar, "situated or operating in the same plane," 1849, from co- + planar.

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