the adverb "stolidly" is built upon the adjective "stolid," which entered english in the 16th century from latin stolidus, signifying dullness, stupidity, or insensitivity. the latin term itself is of obscure origin, though it may be related to stultus (foolish) or derive from a root meaning "to be stunned" or "immovable." the suffix "-ly," of germanic origin, is the standard english adverbial ending. thus, "stolidly" literally means "in a dull or insensitive manner," evolving to describe a demeanor characterized by an unemotional, impassive, or seemingly unresponsive calmness.

Structure: stolid + ly stolid (from latin stolidus, meaning dull, stupid, insensitive; functions as the adjective root) + ly (from old english -līce, forming adverbs from adjectives; functions as an adverb-forming suffix)