Jamieson, Fausset and Brown on James 5:12

12. But above all--as swearing is utterly alien to the Christian meek "endurance" just recommended.

swear not--through impatience, to which trials may tempt you (Jas 5:10, 11). In contrast to this stands the proper use of the tongue, Jas 5:13. James here refers to Mt 5:34, &c.

let your yea be yea--Do not use oaths in your everyday conversation, but let a simple affirmative or denial be deemed enough to establish your word.

condemnation--literally, "judgment," namely, of "the Judge" who "standeth before the doors" (Jas 5:9).