"Knightly deed" refers to a noble or brave act performed by a knight or someone who embodies the qualities of a knight. Synonyms for this phrase could be "chivalrous act," "gallant feat," "valiant action," "heroic undertaking," or "noble quest." These words all represent a sense of honor, valor, and righteousness that is often associated with knights and their code of chivalry. Other possible synonyms could include "courageous mission," "brave adventure," "noble endeavor," or "honorable pursuit." All of these phrases elicit a deep sense of respect and admiration for those who perform such deeds with integrity and bravery.