Etymology
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relevant (adj.)

"to the purpose, applicable, pertinent to the matter at hand," 1550s, from French relevant "depending upon," originally "helpful," from Medieval Latin relevantem (nominative relevans), from stem of Latin relevare "to lessen, lighten," hence "to help, assist; comfort, console" (see relieve). Not generally used until after 1800.

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relevance (n.)

"pertinence, applicableness; recognizable connection," 1733; see relevant + -ance. Related: Relevancy (1560s).

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irrelevant (adj.)

1680s, from assimilated form of in- (1) "not, opposite of" + relevant. Related: Irrelevantly.

It is worth remembering that irrelevant & relieving are the same word; that, presumably, is irrelevant which does not relieve or assist the problem in hand by throwing any light upon it. [Fowler]
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*legwh- 
Proto-Indo-European root meaning "not heavy, having little weight."

It forms all or part of: alleviate; alleviation; alto-rilievo; carnival; elevate; elevation; elevator; leaven; legerdemain; leprechaun; Levant; levator; levee; lever; levity; levy (v.) "to raise or collect;" light (adj.1) "not heavy, having little weight;" lighter (n.1) "type of barge used in unloading;" lung; relevance; relevant; releve; relief; relieve.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit laghuh "quick, small;" Greek elakhys "small," elaphros "light;" Latin levare "to raise," levis "light in weight, not heavy;" Old Church Slavonic liguku, Russian lëgkij, Polish lekki, Lithuanian lengvas "light in weight;" Old Irish lu "small," laigiu "smaller, worse;" Gothic leihts, Old English leoht "not heavy, light in weight."
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pertinency (n.)

"quality of being relevant to the matter in hand," 1590s, from stem of Latin pertinens "pertaining," present participle of perinere (see pertain) + -cy.

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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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recital (n.)

1510s, a legal term, "that part of a deed which contains a rehearsal or statement of relevant facts," from recite (v.) + -al (2). From 1560s as "that which is recited, a story." The meaning "act of reciting, a telling over, narration" is from 1610s; musical performance sense is from 1811 (especially one given by a single performer).

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deft (adj.)

"apt or dexterous, subtly clever or skillful," mid-15c., from Old English gedæfte, which meant "mild, gentle, simple, meek," but which splintered into different forms and senses in Middle English, yielding this word and also daft (q.v.). In Middle English it also could mean "well-mannered, gentle, modest, mild," and "dull, uncouth, boorish." Cognate with Gothic gadaban "to be fit," Old Norse dafna "to grow strong," Dutch deftig "important, relevant," from Proto-Germanic *dab-, which has no certain IE etymology and is perhaps a substratum word. Related: Deftness.

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material (adj.)

mid-14c., "real, ordinary; earthly, drawn from the material world" (contrasted with spiritual, mental, supernatural), a term in scholastic philosophy and theology, from Old French material, materiel (14c.) and directly from Late Latin materialis (adj.) "of or belonging to matter," from Latin materia "matter, stuff, wood, timber" (see matter (n.)).

From late 14c. as "made of matter, having material existence; material, physical, substantial." From late 15c. as "important, relevant, necessary, pertaining to the matter or subject;" in the law of evidence, "of legal significance to the cause" (1580s).

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-ics 
in the names of sciences or disciplines (acoustics, aerobics, economics, etc.), a 16c. revival of the classical custom of using the neuter plural of adjectives with Greek -ikos "pertaining to" (see -ic) to mean "matters relevant to" and also as the titles of treatises about them. Subject matters that acquired their English names before c. 1500, however, tend to be singular in form (arithmetic, logic, magic, music, rhetoric). The grammatical number of words in -ics (mathematics is/mathematics are) is a confused question.
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