Vincent's NT Word Studies
8. A double-minded man is unstable, etc. The A.V. puts this as an independent apophthegm, which is wrong. The sentence is a comment and enlargement upon that man. " Let not that man think," etc., "a doubleminded man, unstable in all his ways." So Rev.Double-minded (diyucov). Peculiar to James, here and ch. iv. 8. Not deceitful, but dubious and undecided.
Unstable (akatastatov). Only here in New Testament. The kindred ajkatastasia, confusion, is found ch. iii. 16, and elsewhere.