in·sou·ci·ance
Pronunciation: \in-ˈsü-sē-ən(t)s, aⁿ-süs-ˈyäⁿs\
Function: noun
Etymology: French, from in- + soucier to trouble, disturb, from Old French, from Latin sollicitare — more at solicit
Date: 1799
: lighthearted unconcern : nonchalance
— in·sou·ci·ant \in-ˈsü-sē-ənt, aⁿ-süs-yäⁿ\ adjective
— in·sou·ci·ant·ly \in-ˈsü-sē-ənt-lē\ adverb

(from a forum post)