Seeming to be coolly unconcerned or indifferent. See synonyms at cool.
[French, from Old French, present participle of nonchaloir, to be unconcerned : non-, non- + chaloir, to cause concern to (from Latin calēre, to be warm, heat up).]
nonchalantly non'cha·lant'ly adv.WORD HISTORY A nonchalant person is not likely to become warm or heated about anything, a fact that is underscored by the etymology of the word nonchalant. It stems from Old French, where it was formed from the negative prefix non- plus chalant, the present participle of the verb chaloir, "to be concerned." This in turn came from the Latin word calēre, which from its concrete sense "to be hot or warm" developed the figurative sense "to be roused or fired with hope, zeal, or anger." French formed a noun nonchalance from the adjective nonchalant that was borrowed into English by 1678; the adjective itself was borrowed later, as it is not attested for another half-century.
