ANSWER: The use of one word with several meanings is because there are more ideas than words in any language. It will help you to know that "flesh" has the signification of sinfulness only in Paul's epistles, who sometimes uses it in a good sense, as when he says "the life that I live in the flesh," meaning his body. Outside of Paul's epistles it means either the living tissue covering the bones, or the body as a whole, or all men when "all flesh" occurs. Paul does not use it as a synonym for sin, but as "proneness to sin," usually to sensuality, but sometimes it includes sins which are independent of the body; such as pride and malice. "Flesh" in the Gospels is not used in the moral sense, but in the physical, as in John 8:4, "That which is born of the flesh is flesh."
— Steele's Answers p. 252.