'smarty-pants' is formed from 'smarty' + 'pants'. 'Smarty' derives from 'smart' with the diminutive/agentive suffix '-y' meaning 'one who is smart or clever'; 'pants' (informal) is used playfully to form a colloquial disparaging compound.

'smart' in Old English (as 'smeart') originally meant 'pain' or 'stinging'; by Middle English it had taken on senses of 'quick, brisk' and later 'clever'. The compound 'smarty-pants' appears in early 20th-century American English as a jocular or mildly pejorative term, with related forms like 'smart aleck' influencing its use.

Initially, 'smart' had non-intellectual senses (e.g. pain, quick); over time it shifted toward 'clever' or 'sharp'. 'Smarty-pants' emerged to label someone as ostentatiously clever or irritatingly knowledgable, a nuance that combines teasing and mild insult.