/ˈmɔːrtəl/
[adjective] Describing a living being that will eventually die, or something that can cause death. It is often used to contrast human beings, who die, with gods or immortal creatures, who live forever.
Every time you say 'mortal,' you are speaking a word built on the same ancient root as 'murder.'
At the heart of 'mortal' lies the Proto-Indo-European root *mer-, meaning 'to die' or 'to rub away.' This ancient root, reconstructed from patterns shared across dozens of languages, captures one of the oldest human ideas: that life wears down and ends. From *mer- came the Latin word mors, meaning 'death,' and its genitive form mortis. From mortis, Latin built the adjective mortalis, meaning 'subject to death' or 'belonging to death.' It is this Latin word, mortalis, that English borrowed directly in the 14th century.
The word arrived in English through Old French mortel, which itself came from Latin mortalis. This was a common pathway for English words in the medieval period — Latin terms passed through the filter of Old French during and after the Norman Conquest of 1066, and then settled into English with slightly altered spelling and sound. By the time English writers were using 'mortal' regularly, it carried both a factual meaning (a being that dies) and a moral or emotional weight, as in 'a mortal sin' — a sin so serious it could bring spiritual death to the soul.
The family of words descended from Latin mors and mortis is surprisingly large and revealing. Latin mors → Old French mort → English 'mort' (an old word for death, now rare). Latin mortalis → Old French mortel → English 'mortal.' Latin immortalis → Old French immortel → English 'immortal.' Latin mortarium (a vessel associated with grinding, but also with the dead weight of heavy stone) → Old French mortier → English 'mortar,' both the building cement and the bowl used for grinding. And Latin mori (to die) → mortificare (to make dead) → Old French mortifier → English 'mortify,' which once meant to kill the flesh through religious discipline and now simply means to feel deeply embarrassed.
Perhaps most surprising is the connection to 'murder.' The Proto-Indo-European root *mer- also produced the Proto-Germanic *murþraz, meaning a killing, which became Old English morþor and then Middle English 'murder.' So 'mortal' and 'murder,' though they sound nothing alike, are ancient cousins — both descended from the same prehistoric word for dying. The same root also fed into 'mortgage': Latin mors + Old French gage (pledge) combined to form Old French mortgage, literally a 'death pledge' — a loan that is 'dead' once it is paid off, or one that traps the borrower until they die paying it.
Over the centuries, 'mortal' expanded its meaning in interesting ways. Beyond its basic sense of 'able to die,' it came to describe anything associated with extreme or fatal intensity. A 'mortal wound' kills you. A 'mortal enemy' is one you fight to the death. 'Mortal terror' is a fear so great it feels like dying. In theology, a 'mortal sin' in Catholic teaching is one that destroys the life of grace in the soul, making it spiritually dead. In everyday speech today, people sometimes use 'mortal' almost playfully — 'mere mortals' contrasted with superhuman experts — showing how a word originally carved out of fear and death can soften over time into something almost affectionate.