The phrase "letting fly" typically means to release or burst forth with great force or intensity, often in reference to physical actions such as throwing, shooting or yelling. Antonyms for this phrase might include words like restrained, controlled, or subdued, which suggest a more deliberate and measured approach. Other antonyms might include gentle, mild, or quiet, which convey a sense of calmness or softness rather than force or power. Ultimately, the antonyms for "letting fly" depend on the specific context and tone of the situation in which the phrase is being used.