i.e. When I abounded in all sorts of blessings; which is oft signified by this or the like phrases, as Genesis 49:11 Deuteronomy 33:24 Job 20:17 Psalms 81:16; when I had such numerous herds of cattle, and consequently such plenty of butter, that if I had needed it, or been pleased so to use it, I might have washed my feet with it; when not only fruitful fields, but even barren and rocky places, (such as that part of Arabia was where Job lived,) yielded me olive trees and oil in great plenty. See Poole on "Deuteronomy 32:13".
Related Commentaries of Job 29:6
Job 29:6
6 When I washed my steps with butter, and the rock poured me out rivers of oil;