my lips will not speak wickedness, and my tongue will not utter deceit.
Job's discourse here is called a parable (mashal), the title of Solomon's proverbs, because it was grave and weighty, and very instructive, and he spoke as one having authority. It comes from a word that signifies to rule, or have dominion; and some think it intimates that Job now triumphed over his opponents, and spoke as one that had baffled them.
Commenting on Job 27:1-6
My lips shall not speak wickedness,.... This is the thing he swears to, this the matter of his oath, not only that he would not speak a wicked word not anything corrupt, unsavoury, unchaste, profane, and idle nor speak evil of his neighbours and friends or of any man; but that he would not speak wickedly of himself, as he must do, if he owned...
(Job 6:28, Job 6:30). The "deceit" would be if he were to admit guilt against the witness of his conscience.