When used with the article “the” before the word, it refers to the fallen sin-nature in man. But when it is used without the article, it usually is denoting what is human, rather than what is sinful. (Unfortunately, the KJV does not always follow this rule, as do the more critical translations produced by J. N. Darby, W. Kelly, etc.)
In keeping with this, Scripture is careful to state that the Lord Jesus came “in flesh”—leaving out the article (1 John 4:2-3; Rom. 1:3; 1 Tim. 3:16). To say, “in the flesh” in connection with the Lord’s humanity could imply that He took part in the fallen sin-nature, which He most definitely did not. However, the article “the” is rightly used in connection with men because all in Adam’s race have been born with a fallen sin-nature (Rom. 7:5, etc.).