fault, n.
First attested in the late 13th century from Old French faute, earlier falte, from Vulgar Latin fallita, "failing, shortcoming", which grew out of Classical Latin falsus, the past participle of fallere, "to deceive or disappoint", which is also the source of Modern English fail, fallible, and false (through falsus).
Originally this word was borrowed directly from French in both spelling and pronunciation. The medial -l- was added in the 1500s as part of the trend of spelling reform, in an attempt to imitate the original Latin. It did not begin to be pronounced, however, until the 1700s.
This word gained the sense of "physical imperfection" in the early 14th century, the sense of "moral failing" in the late 14th century, and is first attested as a verb in the mid 15th century. It appears as jargon in the sport of tennis from ca. 1600, most closely resembling the etymological sense of "a deception", and finally in the geological sense, from 1796.
