Quiz: Remember the definition of mal de mer?
1734, from French nonchalant, present participle of nonchaloir "be indifferent to, have no concern for" (13c.), from non- "not" (see non-) + chaloir "have concern for," ultimately from Latin calere "be hot" (see calorie). French chaland "customer, client" is of the same origin. Related: Nonchalantly.