Jamieson, Fausset and Brown on 2 Corinthians 11:17

17. not after the Lord--By inspired guidance he excepts this "glorying" or "boasting" from the inspired authoritativeness which belongs to all else that he wrote; even this boasting, though undesirable in itself, was permitted by the Spirit, taking into account its aim, namely, to draw off the Corinthians from their false teachers to the apostle. Therefore this passage gives no proof that any portion of Scripture is uninspired. It merely guards against his boasting being made a justification of boasting in general, which is not ordinarily "after the Lord," that is, consistent with Christian humility.

foolishly--Greek, "in foolishness."

confidence of boasting--(2Co 9:4).