Etymology
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false (adj.)

late Old English, "intentionally untrue, lying," of religion, "not of the true faith, not in accord with Christian doctrines," from Old French fals, faus "false, fake; incorrect, mistaken; treacherous, deceitful" (12c., Modern French faux), from Latin falsus "deceptive, feigned, deceitful, pretend," also "deceived, erroneous, mistaken," past participle of fallere "deceive, disappoint," which is of uncertain origin (see fail (v.)).

Adopted into other Germanic languages (cognates: German falsch, Dutch valsch, Old Frisian falsk, Danish falsk), though English is the only one in which the active sense of "deceitful" (a secondary sense in Latin) has predominated. From c. 1200 as "deceitful, disloyal, treacherous; not genuine;" from early 14c. as "contrary to fact or reason, erroneous, wrong." False alarm recorded from 1570s. False step (1700) translates French faux pas. To bear false witness is attested from mid-13c. False prophet "one who prophecies without divine commission or by evil spirits," is attested from late 13c.

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falsies (n.)
"padded brassiere," 1943, from false + -ie.
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falseness (n.)
c. 1300, "deceitfulness, treachery, faithlessness, dishonesty," from false + -ness.
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falsely (adv.)
c. 1200, "with intent to deceive, deceitfully," from false + -ly (2). From c. 1300 as "wrongly; untruthfully;" early 14c. as "incorrectly."
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faux pas (n.)
"breach of good manners, any act that compromises one's reputation," 1670s, French, literally "false step." See false and pace (n.).
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falsetto (n.)
"artificially high voice," 1774, from Italian falsetto, diminutive of falso "false," from Latin falsus (see false). Earlier in an Englished form as falset (1707). One who sings thus is a falsettist.
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faux (adj.)
from French faux "false" (12c., see false). Used with English words at least since 1676 (Etheredge, faux-prude). Used by itself, with French pronunciation, from 1980s to mean "fake."
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falsify (v.)

mid-15c., falsifien, "to prove false," from Old French falsifier "to falsify, counterfeit" (15c.), from Late Latin falsificare "make false, corrupt," from Latin falsus "erroneous, mistaken" (see false). Meaning "to make false" is from c. 1500. Earlier verb was simply falsen (c. 1200). Related: Falsified; falsifying.

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falsity (n.)
c. 1300, "deceitfulness, treachery, dishonesty," from Old French fauseté "falsehood" (12c., Modern French fausseté), from Late Latin falsitatem (nominative falsitas), from Latin falsus "erroneous, mistaken" (see false). From late 14c. as "untrue statement or doctrine;" from 1570s as "character of being not true."
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falsehood (n.)
c. 1300, falshede, "deceitfulness," also "a lie; that which is false," from false + -hood. Formed on the same pattern are Old Frisian falschede, Dutch valschheid, German Falschheit, Swedish falskhet. Former noun forms in English, now extinct, included falsage "wrongdoing" (late 15c.), falsdom "deceitfulness, treachery; a lie" (c. 1300), fals-lek "falsehood" (early 14c.), falsshipe "deceitfulness, dishonesty" (c. 1200).
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