"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.
-ance
word-forming element attached to verbs to form abstract nouns of process or fact (convergence from converge), or of state or quality (absence from absent); ultimately from Latin -antia and -entia, which depended on the vowel in the stem word, from PIE *-nt-, adjectival suffix.
Latin present-participle endings for verbs stems in -a- were distinguished from those in -i- and -e-. Hence Modern English protestant, opponent, obedient from Latin protestare, opponere, obedire.
As Old French evolved from Latin, these were leveled to -ance, but later French borrowings from Latin (some of them subsequently passed to English) used the appropriate Latin form of the ending, as did words borrowed by English directly from Latin (diligence,absence).
English thus inherited a confused mass of words from French (crescent/croissant), and further confused it since c. 1500 by restoring -ence selectively in some forms of these words to conform with Latin. Thus dependant, but independence, etc.
*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."