Longman Dictionary of Contemporary English
Wiktionary
n. (context linguistics idiomatic English) A word in a language that bears a deceptive resemblance to a word in another language but in fact has a different meaning. The words in question may well be etymologically related, but in such cases semantic shifts have made them drift apart.
Wikipedia
False friends are words in two languages (or letters in two alphabets) that look or sound similar, but differ significantly in meaning. An example is the English embarrassed and the Spanish embarazada (which means pregnant), or the word sensible, which means reasonable in English, but sensitive in French, German and Spanish.
The term is a shortened version of the expression "false friend of a translator", the French version of which (faux amis du traducteur) was introduced by linguists Maxime Kœssler and Jules Derocquigny in 1928, in the book Les Faux Amis ou les trahisons du vocabulaire anglais (False Friends, or the Pitfalls of the English Vocabulary, with a sequel, Autres Mots anglais perfides).
There is often a partial overlap in meanings, which creates additional complications.
Similar words may also fail to catch all the nuances of each word in both languages. For instance, the French demande simply means a 'request', which is similar to but also very different from a demand in English and demandar in Spanish ("to sue").
The two "false friends" may actually be related, having the same origin, unlike " false cognates", which are similar words in different languages that appear to have a common historical linguistic origin (whatever their current meaning) but actually do not.
As well as producing completely false friends, the use of loanwords often results in the use of a word in a restricted context, which may then develop new meanings not found in the original language. For example, angst means "fear" in a general sense (as well as "anxiety") in German, but when it was borrowed into English in the context of psychology, its meaning was restricted to a particular type of fear described as "a neurotic feeling of anxiety and depression". Also, gymnasium meant both 'a place of education' and 'a place for exercise' in Latin, but its meaning was restricted to the former in German and to the latter in English, making the expressions into false friends in those languages as well as in Greek, where it started out as 'a place for naked exercise'.
Usage examples of "false friend".
If she was trusted by Old Man, she was, in one sense, a false friend.
So, he caused his false friend, Athelwold, to be murdered in a wood, and married his widow, this bad Elfrida.
The finding of the body of Golden Dase, a thief and false friend to Jimmy, revealed that the Nighthawks had indeed infiltrated the Mockers.