Faulty means having something wrong with it — a faulty switch, faulty wiring, the goods were faulty. It is the standard word for a manufactured thing that does not work as it should.
It matters in consumer law: returning something because it is faulty gives you rights that changing your mind does not, which is why the word appears on every returns policy. Applied to reasoning it means unsound: faulty logic, faulty assumptions. The noun is a fault, and the important pair is faulty, meaning defective, against at fault, meaning to blame. A faulty brake is broken; the driver at fault caused the crash.
1adjectivefaultyNot working properly because something is wrong with it.
“The heater was faulty and had to go back.”
2adjectivereasoningOf reasoning, containing a mistake.
“The whole argument rests on a faulty assumption.”
faulty is a defective state.
Latin
falsus
Vulgar Latin
*fallita — shortcoming
Old French
faute
Anglo-Norman
faute
Middle English
faulte
English
fault
English, today
faulty
Built from
From fault + -y. fault comes from Middle English faulte, 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 (“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.
Etymology from Wiktionary, CC BY-SA