Antonyms of the phrase "look up and down" could include avoiding eye contact, glancing casually, or giving a quick once-over. These behaviors might suggest disinterest, lack of curiosity or simply wanting to avoid drawing attention. Conversely, synonyms for "look up and down" could include inspecting closely, scrutinizing or examining closely, all of which imply a deep level of interest or scrutiny. Using words like "appraise," "analyze," or "assess" might also capture the sense of a concentrated gaze with the intent of deriving important information from the subject being looked at.