The word "struts stuff" is an idiom that refers to someone who acts or behaves arrogantly, confidently, or showily. Antonyms for this phrase would include words such as "humbles oneself," "downplays," "shows modesty," "is reticent," or "acts with reservation." To put it simply, the opposite of "strutting one's stuff" would involve acting more reserved, humble, and modest rather than showing off, boasting, or drawing attention to oneself. This can be an important trait to cultivate in many situations, particularly in professional settings where showing too much confidence can be seen as a negative rather than a positive attribute.