The opposite or antonym for "picked up check" may vary depending on the context in which the phrase is used. However, some possible antonyms include "left behind check," "dropped check," or "ignored check." These antonyms suggest that the check was not taken or acknowledged in some way, but rather neglected or forgotten. Another possible antonym could be "returned check," which means the check was not kept or cashed, but returned to the sender. Overall, the antonym for "picked up check" implies a lack of action or attention towards the check, either intentionally or due to neglect.