Etymology
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incidental (adj.)

"casual, occurring casually in connection with something else; of minor importance," 1640s, from Medieval Latin incidentalis, from incidens (see incident (n.)). The earlier adjective in this sense was incident (1520s). Incidentals (n.) "'occasional' expenses, etc.," is attested by 1707. Incidental music "background music," originally in operas, is from 1812.

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incidentally (adv.)

1520s, "by the way, casually;" see incidental + -ly (2). Sense of "as a new but related point" attested by 1925.

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side-light (n.)

also sidelight, c. 1600, "light coming from the side," from side (adj.) + light (n.). The figurative meaning "incidental information on a subject, an oblique illustration" is attested by 1862.

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circumstantial (adj.)

c. 1600, "attending, incidental," also "derived from circumstances," from Latin circumstantia (see circumstance) + -al (1). Related: Circumstantially. Legalese circumstantial evidence "evidence from more or less relevant circumstances bearing upon a case," as distinguished from direct testimony, is attested by 1691.

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incidence (n.)

early 15c., "incidental matter," from Old French incidence (15c.), from Late Latin incidentia, from incidere "to happen, befall" (see incident (n.)). Meaning "act of coming into contact with or affecting" is from 1650s. In physics, of rays of light, etc., considered with reference to direction, from 1620s.

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occurrence (n.)

"that which presents itself, that which happens without design or expectation," 1530s, from French occurrence "unexpected happening" or directly from Medieval Latin occurrentia, from Latin occurentem (nominative occurens), present participle of occurrere (see occur). The adjective occurrent "occurring, happening, incidental" (mid-15c.) is long obsolete.

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casualty (n.)

early 15c., casuelte, caswelte, "chance, accident; incidental charge," from casual (adj.) on the model of royalty, penalty, etc. From the earliest use especially of untoward events or misfortunes. The meaning "losses in numbers from a military or other troop" is from late 15c. The meaning "an individual killed, wounded, or lost in battle" is from 1844. Casuality had some currency 16c.-17c. in the sense "chance, a chance occurrence," especially an unfortunate one, but now is obsolete.

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coincidence (n.)

c. 1600, "exact correspondence in substance or nature," from French coincidence, from coincider, from Medieval Latin coincidere, literally "to fall upon together," from assimilated form of  Latin com "with, together" (see com-) + incidere "to fall upon" (from in- "upon" + combining form of cadere "to fall," from PIE root *kad- "to fall").

From 1640s as "occurrence or existence during the same time." Meaning "a concurrence of events with no apparent connection, accidental or incidental agreement" is from 1680s, perhaps first in writings of Sir Thomas Browne.

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perquisite (n.)

mid-15c., "property acquired other than by inheritance" (c. 1300 in Anglo-Latin), from Medieval Latin perquisitum "thing gained, profit," in classical Latin, "thing sought after," noun use of neuter past participle of perquirere "to seek, ask for," from per "thoroughly" (see per) + quærere "to seek" (see query (v.)). For Latin vowel change, see acquisition. The general meaning "any incidental profit, gain, or fee on top of regular wages" is by 1560s.

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mention (n.)

c. 1300, mencioun, "a note, a reference, a calling to mind by speech or writing," from Old French mencion "mention, memory, speech," from Latin mentionem (nominative mentio) "a calling to mind, a speaking of, a making mention," from root of Old Latin minisci "to think," related to mens (genitive mentis) "mind," from PIE root *men- (1) "to think." From late 15c. as "statement about or in reference to a person or thing," which by mid-18c. had diminished to "incidental or casual reference," though in military use a mention in the dispatches remained an important thing.

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