What did the word 'nice' originally mean?
It came from Latin 'nescius', and its meaning changed over centuries.
English took 'nice' from Old French, which got it from the Latin 'nescius', 'ignorant' (from 'ne-', not, plus 'scire', to know). The same root gives the rare English word 'nescience'. By the end of the 1300s the word was already showing several meanings, which makes the earliest English sense hard to pin down; 'foolish' or 'ignorant' is generally given as the first.
The meaning then drifted a long way. Over the following centuries 'nice' was recorded in senses including 'wanton' or 'dissolute', 'coy' or 'shy', 'fussy' or 'hard to please', 'delicate' or 'subtle', and 'precise'. Merriam-Webster cites uses such as 'nice' meaning 'demanding great precision' in an early 20th-century trade magazine, and traces even a sense of 'chaste' that survived into the 20th century. Traces of the older meanings remain in phrases like 'a nice distinction' or 'a nice point', where 'nice' means subtle or exact.
The modern 'agreeable, pleasant' sense is the most common today and developed gradually, with the older meanings fading. Because the word shifted over so many centuries, historians of language often use it as a standard example of semantic change, where a term moves from a negative sense to a positive one. Dating each shift precisely is difficult, so claims about exactly when 'nice' became kind are best treated as approximate.