the word "unimportant" is a straightforward english formation, combining the native english negative prefix "un-" with the adjective "important." the prefix "un-" derives from old english "un-," which has a direct lineage from the proto-germanic un- and ultimately from the proto-indo-European n̥-, meaning "not." the core adjective "important" entered english from middle french "important," which itself came from medieval latin "important-," the present participle stem of "importare," meaning "to be of consequence, to signify." the literal latin meaning of "importare" was "to bring in, carry in" (from "in-" + "portare," "to carry"), but its sense evolved in medieval latin to signify "to imply, be of consequence." therefore, "unimportant" logically constructs its meaning from its parts: "not" + "of consequence."

Structure: un (not) + important (significant) un- (old english, a prefix meaning "not"); important (from latin through french, meaning "significant, of consequence").