Jamieson, Fausset and Brown on Ecclesiastes 5:6

6. thy flesh--Vow not with "thy mouth" a vow (for example, fasting), which the lusts of the flesh ("body," Ec 2:3, Margin) may tempt thee to break (Pr 20:25).

angel--the "messenger" of God (Job 33:23); minister (Re 1:20); that is, the priest (Mal 2:7) "before" whom a breach of a vow was to be confessed (Le 5:4, 5). We, Christians, in our vows (for example, at baptism, the Lord's Supper, &c.) vow in the presence of Jesus Christ, "the angel of the covenant" (Mal 3:1), and of ministering angels as witnesses (1Co 11:10; 1Ti 5:21). Extenuate not any breach of them as a slight error.