fault (n.) Look up fault at Dictionary.com
late 13c., faute, "deficiency," from Old French faute (12c.) "opening, gap; failure, flaw, blemish; lack, deficiency," from Vulgar Latin *fallita "a shortcoming, falling," noun use of fem. past participle, 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 was not pronounced till 18c. Sense of "physical defect" is from early 14c.; that of "moral culpability" is first recorded late 14c. Geological sense is from 1796. The use in tennis (c.1600) is closer to the etymological sense.
fault (v.) Look up fault at Dictionary.com
late 14c., Scottish, "be deficient;" see fault (n.). Meaning "find fault with" is from mid-15c. Related: Faulted; faulter; faulting.