Part of speech: Adjective (satellite)
marked by blithe unconcern
Example: an ability to interest casual students; showed a casual disregard for cold weather; an utterly insouciant financial policy; an elegantly insouciant manner; drove his car with nonchalant abandon; was polite in a teasing nonchalant manner
Casual, Insouciant, Nonchalant
The word "nonchalant" comes from Latin non, which comes from Old French nonchaloir, which was borrowed from French nonchalant.
Roman military conquests permanently planted Latin vocabulary across three continents.