Here a person's "mouth" represents a person's speech, and the person himself is represented by his "flesh." Alternate translation: "Do not let what you say cause you to sin" (See: [[rc://en/ta/man/translate/figs-metonymy]] and [[rc://en/ta/man/translate/figs-synecdoche]])
The author uses this rhetorical question to emphasize that it is foolish to make a vow that you will not keep. This question can be written as a statement. Alternate translation: "It would be foolish to make God angry by vowing falsely, provoking God to destroy the work of your hands." (See: [[rc://en/ta/man/translate/figs-rquestion]])