What's the difference between a "false cognate" and a "false friend"?

Question

There are two terms used for pairs of words (in the same or different languages) that look similar but are actually unrelated: false friend and false cognate. Are these terms synonymous? If not, what's the difference?

Answer 1

They are distinct.

Definitions

• False cognates are words that are similar in their modern forms despite having different etymologies. This is regardless of whether the modern meanings are similar.
• False friends are words that are similar in their modern forms despite having different modern meanings. This is regardless of whether the words are etymologically connected.

Examples

True cognates, true friends

Words with a common etymology and modern form and meaning. These are extremely numerous, of course.

• Banana in numerous languages

True cognates, false friends

Words with a common etymology but which have shifted to have different meanings.

• Embarrassed 'feeling publicly shamed' in English, and embarazado 'pregnant' in Spanish
• /susi/ sushi 'dish containing sushi rice' in Japanese, and sushi 'prepared raw fish' in colloquial English; in this case the meaning shifted metonymically when it was borrowed
• smoking 'inhaling tobacco smoke' in English, and smoking 'tuxedo' in Czech/Danish/French/Italian/Polish/Serbo-Croatian/Spanish, another metonymic borrowing

False cognates, true friends

Words etymologically unrelated that nevertheless now have similar meanings (typically by coincidence, but sometimes by influence/reinforcement).

• Ancient Greek /tʰea/ versus Latin /dea/ (differing in voicing), both meaning 'goddess', are true friends but false cognates. In meaning, both relate to deities, but they actually derive from different Indo-European roots (*/dhes/- and */deiwós/ respectively).
• Portuguese obrigado and Japanese arigatoo both mean 'thank you'. Due to Portuguese contact with Japan a borrowing would be plausible, but in fact they're etymologically unrelated.. In fact the word arigatoo is attested since the 8th century as a compound from Japanese roots, and meaning 'thanks' since the 15th century, both before the Portuguese reached Japan.
• Within a language, suppletive verb forms may be this; for example English 'be' and its forms
• English: Thomas Crapper, popularizer of the flush toilet, and crapper 'toilet'. Presumably Crapper's name is not actually derived from crap 'feces', and -er is a common derivational suffix for 'thing which is used to ...', but having his brand name placed on toilets likely reinforced the use of crapper.
• According to Wikipedia, Finnish tytinä 'wobbliness' looks like a derivation from tutista 'to wobble' (Finnish /u/ and /y/ alternate due to vowel harmony), but is a actually derived instead from Russian stúden'.
• English: cash (money), from Middle French caisse; cash (Chinese coin), reportedly from Tamil
• dog 'dog' in Mbabaram (an Australian Aboriginal language) and dog in English. The Mbabaram word is "pronounced almost identically to the English word".

False cognates, false friends

These are completely unrelated words that happen to look similar:

• Greek κάππα 'the letter κ' and Japanese /kaʔ̩pa/ (transliterated kappa) 'water sprite'
• Portuguese galinha [ɡəˈɫi.ɲə] 'hen' and English galena [ɡəɫˈi.nə] 'PbS'

Complications

• Reinforcement and language contact. In the case of false cognates/true friends, if the two languages are in contact, or it occurs within a single language), the similar meanings tend to reinforce each other. They will possibly even be re-analyzed as forms of the same word, thereby merging the lexemes.
• False recent cognates that are true cognates more distantly. Example: Malay nama 'name' might look like a loanword from English name (Malaya was a British colony), but it is actually an older loanword from Sanskrit (cognate to the English via Proto-Indo-European).
• Re-analysis: false-cognate words may have their spelling altered to match the form of a true friend in the other language. English examples: indict, victuals, from French but with elided Latin consonants re-added. Spelling and pronunciation are borrowed via different paths, so the spelling could be considered cognate though the pronunciation is not. (In this case, both are more distantly cognate via Latin; the French words were sound-changed.)
• Calques or loan-translations: the form of the word is cognate, but its components are often not. Example: Latin insecta, Ancient Greek ἔντομον 'insect', from different roots meaning 'cut'

Answer 2

Most often, the expression “false cognate” is used as a synonym for “false friend”. If you google with them, you will mostly find pages that use them synonymously.

However, other meanings have also been proposed. In Concise Encyclopedia of Semantics edited by Keith Allan, the article “False friends”, p. 308–309, describes false cognates as a special case of false friends, namely false friends that are not etymologically related. And in a Unilang discussion, it was suggested (without references) that “false cognates are words that look the same and have the same meaning but have different roots”, so that false cognates would not be false friends at all.

Thus, the expression “false cognate” is best avoided, since it has different meanings to different people. When you encounter it, assume that it probably means “false friend”, but with some suspicion: it might mean something rather different.