From Anstey’s Doctrinal Definitions:
This expression is found in Romans 6:6, Ephesians 4:22, and Colossians 3:9. It, like the “new man,” is an abstract term that describes the corrupt state of the fallen race of Adam—its depraved moral character. The “old man” is not Adam personally, but what is characteristic of Adam’s fallen race. It is the embodiment of every ugly feature that marks the race. To see the old man properly we must look at the race as a whole, for it is unlikely that any one person would be marked by all the ugly features that characterize that corrupt state. For instance, one person in the fallen race may be characterized by being angry and deceitful, but he may not be immoral. Another person may not be known for losing his temper, nor for being deceitful, but he is terribly immoral. However, taking the race as a whole, we see all the ugly features that compose the old man.
Romans 6:6 and Romans 8:3 state that God has judged the “old man” at the cross of Christ. And, Ephesians 4:22 and Colossians 3:9 tell us that it is something the believer has “put off” in receiving Christ as his Saviour. As part of the Christian stand, we have by our profession, confessedly put off everything to do with that corrupt state. This putting off is stated in the aorist tense in the Greek—that is, having done it once for all time. Hence, as Christians, we are confessedly no longer associated with it. Unfortunately, the KJV translates Ephesians 4:22-24 as an exhortation, making the putting off of the old man something that we are to do in our lives as a daily thing. But in reality, the putting off of the old man is something that the believer does once and for all when he takes his stand with Christ. The passage should read: “Having put off according to the former conversation, the old man ... ” (J. N. Darby Trans.).
The "old man" is often used synonymously with the old nature (the flesh) in the believer. This is a widespread misunderstanding among Christians. They will say things such as: “The old man in us desires things that are sinful.” Or, “Our old man wants to do this or that evil thing ... ” However, these statements are confusing the old man with the flesh. Scripture does not use the term in that way. Mr. Darby remarked, “The old man is being habitually used for the flesh incorrectly” (Food for the Flock, vol. 2, p. 286). One difference is that the old man is never said to be in us, while the flesh most certainly is. F. G. Patterson said, “Nor do I find that Scripture will allow us to say that we have the old man in us—while it teaches most fully that we have the flesh in us” (A Chosen Vessel, p. 51). Hence, it is not accurate to speak of the old man as being a thing living in us with appetites, desires, and emotions, as does the flesh. H. C. B. G. said, “I know what a Christian means who loses his temper, and says it is ‘the old man,’ yet the expression is wrong. If he said it was ‘the flesh,’ he would have been more correct” (Food for the Flock, vol. 2, p. 287). If the old man were the flesh, then Ephesians 4:22-23 (in the KJV) would be telling us that we need to put off the flesh! However, there is no exhortation in Scripture to put off the flesh. It is something that won’t happen until we die, or when the Lord comes.
Thus, the "old man" has been judged at the cross and has been put off by the believer in receiving Christ as his Saviour. While there is no exhortation in Scripture to put off the old man, there is an exhortation to “put off” those things that may be in our lives that characterize the old man (Col. 3:8-9). Nor is there an exhortation in Scripture for Christians to “reckon the old man dead,” as people often say. This mistaken idea supposes that it is some evil thing living in us (i.e. the flesh). Scripture says that we are to “reckon ourselves to be dead indeed unto sin” (Rom. 6:11). Others speak of the old man as being dead. This is a misunderstanding too. Again, it suggests that it was once something living in the believer, but has died.
Seven things the “old man” is not:
• It is not Adam personally.
• It is not the flesh in the believer.
• It is not our old position before God.
• It is not synonymous with the first man.
• It is not something that needs to be put to death, or has died.
• It is not something that the believer puts off daily.
• It is not something that we bury in baptism.