A word for the inner life
Today mental is the ordinary English adjective for what belongs to the mind. It covers thought, memory, calculation, mood, and the professional language of psychiatry. A plan can be mental in the sense that it exists only as an idea. A task can be mental when it asks for attention rather than lifting. In medical English the same word has long sat in front of health, illness, patient, and hospital, naming conditions and institutions rather than intellect in the abstract.
In everyday British, Irish, Australian, and New Zealand speech the adjective can also mean wild, furious, or simply a lot of fun. Those later tones sit uncomfortably beside clinical uses, which is why dictionaries now flag some of them as slang, dated, or offensive. None of that later colour is the oldest English meaning. There is also a second, unrelated adjective in anatomy for the chin, but the word people mean in ordinary talk is the one about thought.
How English acquired it
The origin of the word mental is a learned borrowing, not a native Germanic adjective. It is first visible in English in the early fifteenth century. The best-dated evidence points to about 1422, in the writing of Thomas Hoccleve, the London poet and clerk. Sources do not agree on a single immediate donor. Some treat the English form as taken from Middle French mental, itself a scholarly lift from Late Latin. Others allow a direct path from Late Latin mentalis. One standard American dictionary sends the word through Old French instead. The practical picture is the same: late medieval English writers adopted a Latin-style adjective that Romance scholars were already using, rather than building a new one from the old English word for mind.
Late Latin mentalis is transparent. It is Latin mens, genitive mentis, meaning mind, understanding, or reason, plus the usual adjectival suffix -alis. English later respells that suffix as -al, the same ending in dental, legal, and cordial. The root syllable ment- is what English speakers still hear. The word did not grow inside English from mind plus a native ending; the whole adjective arrived already formed.
In Middle English the new word already meant of or pertaining to the intellect. It could also shade toward the soul and the spiritual, a range that later English would split among mental, spiritual, and psychological. That early spiritual colour faded as the language found other adjectives for the soul. What remained as the core is still the Late Latin idea: of the mind, as opposed to the body or to brute matter.
A thinking root, not a folk story
Latin mens belongs to a wide Indo-European family. The reconstructed Proto-Indo-European root is *men-, to think, with a cluster of nouns and verbs for states of mind. From that root come Sanskrit manas, mind or spirit; Greek mania and mantis, the language of frenzy and of the seer; Lithuanian mintis, thought; and Slavic words for memory, such as Russian pamyat. Latin kept several branches of the same stock: memini, I remember; mentio, which gives English mention; and the later medical family around dementia.
English mind is not a child of mental. It is a cousin. Old English gemynd meant memory, remembrance, and the conscious intellect; it is the native Germanic inheritance of the same *men- root, with relatives in Gothic gamunds. So the everyday pair mental and mind looks like a derivation inside English, but the history is different. One word arrived as a Latinate adjective in the 1400s. The other had been in the language since Old English. Related learned vocabulary — mentality, mentally, comment, reminiscent, mentor, admonish — came in by other Latin and French routes, not by growing off the English adjective after 1422.
There is a trap for the eye. Anatomy uses a second adjective mental for the chin, as in mental nerve and mental foramen. That word is dated to about 1727 and comes from Latin mentum, chin, plus the same -al. Mentum belongs to a different Indo-European root, the one behind words for projecting things such as mountains and the mouth. The two English adjectives are homonyms: they look and sound alike, and they share a suffix, but they do not share a history of meaning.
From intellect to institution, then to slang
By the 1520s English could use mental for something done entirely in the head, the ancestor of mental arithmetic and the mental image. That sense is still the most neutral, and it is the one that survives in classroom and office talk without stigma.
A later layer is medical and administrative. Mental health is recorded from the beginning of the nineteenth century; mental illness follows within two decades. Mid-century English then formed mental patient; mental hospital appears near the end of that century. Those compounds treated the adjective as a classification of disorder and of the places meant to treat it. From that institutional use, by 1927, the bare word could mean deranged or crazy in colloquial speech. The jump is easy to see: a label that had described a ward or a diagnosis was applied to the person. In some varieties it later loosened again into extreme or brilliantly chaotic, especially of parties, driving, or sport. Clinical style has since preferred psychiatric, psychological, or more specific diagnoses, partly because the older attributive uses can stigmatize.
The slang path is therefore not a separate coinage. It is a narrowing and then a loosening of the medical attributive, which itself was a narrowing of the older intellectual sense. The spiritual Middle English use dropped away; the chin homonym never mixed with this story at all.
The native cousin worth keeping in view
If one fact should stick, it is the relationship with mind. Speakers often feel that mental is simply the adjective of mind, formed in English the way a Latinate pair looks regular on the page. The chronology says otherwise. Mind was already old when Hoccleve wrote. Mental was imported to do learned and then medical work, and only afterwards drifted into slang. The chin word is a later look-alike from a different root. The thinking adjective and the native noun still meet, honestly, only at the Indo-European root that meant to think.