(1) Put off (apothesthai) means “to remove as one takes off clothes.” The verb is an aorist middle infinitive which indicates three things: (a) this is something we must do for ourselves, an action only we can carry out; (b) it is to be a once-and-for-all kind of action, “put off that old life once and for all, and never go back to it”; and (c) the infinitive form is used to denote the substance of what they had been taught, that which they were to put away, the old life with its old ways of thinking and acting.
(2) Put on (endusasthai) contrasts with the putting off. This word is often used of putting on a garment and is the positive side of verse 22.
Discussion: With this explanation in mind, what does it mean to put off the "old man" and to put on the "new man"?
Source: Galatians and Ephesians, Adult Teacher's Insights, page 68.