Nice used to mean foolish
In medieval English, calling someone nice was no compliment.
The word came into English from Old French nice, which meant simple or foolish. That in turn came from Latin nescius, not knowing, built from ne, not, and the stem of scire, to know, the same root that gives us science. So nice began, quite literally, as ignorant.
Then it kept moving. Before 1300 it could mean timid or faint-hearted. By the late 1300s it meant fussy or fastidious, and by around 1400 dainty or delicate. In the 1500s it took on the sense of precise or careful. The meaning agreeable or delightful is recorded from 1769, and kind or thoughtful from 1830.
The careful sense hasn't disappeared. A nice distinction is a fine, precise one, not a friendly one, and nice and early keeps a trace of it too.
Words rise, fall, narrow and widen
Linguists call this semantic change. Nice is a case of amelioration, a word's meaning improving over time. The reverse, pejoration, happened to silly, which once described a person who was happy and fortunate and now means foolish. Awful once meant full of awe, inspiring wonder or fear, and now usually just means very bad.
Two words for a young male went opposite ways. Knave and knight both once meant boy. Knave sank to servant and then to a deceitful, despicable man, while knight rose to mean a nobleman.
Meanings can also narrow. In Old English, dēor meant a wild animal of any kind. By the end of the Middle English period, around 1500, deer had narrowed to its modern sense. Its relatives elsewhere kept the broad one: German Tier and Dutch dier still simply mean animal.
No one decided any of this. Each change happened as speakers used a word slightly differently from the generation before them, until the new use became the normal one. The same thing is happening to some of today's words, too slowly to notice from the inside.
What did the Latin word behind nice, nescius, mean?
Word meanings drift over the centuries, sometimes into their opposite, and old senses often survive in fixed phrases.
