Fault
Middle English faut(e) ‘lack, failing’, from Old French, based on Latin fallere ‘deceive’. The -l- was added (in French and English) in the 15th century to conform with the Latin word, but did not become standard in English until the 17th century, remaining silent in pronunciation until well into the 18th.
wiktionary
From Middle English faute, faulte, from Anglo-Norman faute, Old French faute, from Vulgar Latin *fallita(“shortcoming”), feminine of *fallitus, in place of Latin falsus, perfect passive participle of fallō(“deceive”). Displaced native Middle English schuld, schuild(“fault”) (from Old English scyld(“fault”)), Middle English lac(“fault, lack”) (from Middle Dutch lak(“lack, fault”)), Middle English last(“fault, vice”) (from Old Norse lǫstr(“fault, vice, crime”)). Compare French faute(“fault, foul”), Portuguese falta(“lack, shortage”) and Spanish falta(“lack, absence”). More at fail, false.
etymonline
fault (n.)
late 13c., faute, "deficiency," from Old French faute, earlier falte, "opening, gap; failure, flaw, blemish; lack, deficiency" (12c.), from Vulgar Latin *fallita "a shortcoming, falling," from Latin falsus "deceptive, feigned, spurious," past participle of fallere "deceive, disappoint" (see fail (v.)).
The -l- was restored 16c., probably in imitation of Latin, but the letter was silent until 18c. Sense of "physical defect" is from early 14c.; that of "moral culpability" (milder than sin or vice, but more serious than an error) is first recorded late 14c. Geological sense is from 1796. The use in tennis (c. 1600) is closer to the etymological sense.
fault (v.)
"find fault with," mid-15c. from fault (n.). Earlier it was used in an intransitive sense of "be deficient" (late 14c., Scottish). Related: Faulted; faulter; faulting.
