When we talk about antonyms for the phrase "signed for," there are a few different possibilities. One option could be "unsigned," meaning that something has not been officially acknowledged or accepted. Another potential antonym is "rejected," which implies that a signed item has been turned away or denied. Additionally, "unclaimed" might be considered an antonym of "signed for," as it implies that something has been sent to a recipient but not yet retrieved. Finally, "undelivered" could also be considered an antonym of "signed for," suggesting that something has yet to be delivered or received by the intended recipient.