"Knocking for loop" is not a commonly used phrase in the English language, and therefore, there are no set antonyms for it. However, we can provide antonyms for the individual words in the phrase. Antonyms for "knocking" could be "praising," "complimenting," or "applauding." Antonyms for "for" could be "against," "opposing," or "versus." Lastly, antonyms for "loop" could be "straight line," "direct path," or "one-way route." It is important to note that antonyms for a phrase depend on the context and the intent of the phrase, and in the case of "knocking for loop," it is highly unlikely that a direct antonym exists.