Mortification refers in Christian theology to the subjective experience of Sanctification, the objective work of God between justification and glorification. Literally it means the ‘putting to death’ of sin in a believer’s life. (Colossians 3:5) Reformed theologian J.I. Packer describes it in the following way: “The Christian is committed to a lifelong fight against the world, the flesh and the devil. Mortification is his assault on the second.”  Christians believe that this internal work against sin is empowered by the Holy Spirit and so therefore is also part of regeneration.
Historical Interpretations of Mortification
Roman Catholic theology frames mortification within the believer’s personal struggle against sin. According to the Catholic Encyclopedia, “What it slays is the disease of the soul, and by slaying this it restores and invigorates the soul’s true life.”
Calvinism and Reformed theology
John Calvin observed that if believers died with Jesus then He destroys our sinful earthly members and their lust, “so that they may no longer perform their functions.” Mortification in Reformed theology has been generally understood to be the subjective experience of sanctification.
sin (n.) Old English synn "moral wrongdoing, injury, mischief, enmity, feud, guilt, crime, offense against God, misdeed," from Proto-Germanic *sun(d)jo- "sin" (cognates: Old Saxon sundia, Old Frisian sende, Middle Dutch sonde, Dutch zonde, German Sünde "sin, transgression, trespass, offense," extended forms), probably ultimately "it is true," i.e. "the sin is real" (compare Gothic sonjis, Old Norse sannr "true"), from PIE *snt-ya-, a collective form from *es-ont- "becoming," present participle of root*es- "to be" (see is). The semantic development is via notion of "to be truly the one (who is guilty)," as in Old Norse phrase verð sannr at "be found guilty of," and the use of the phrase "it is being" in Hittite confessional formula. The same process probably yielded the Latin word sons (genitive sontis) "guilty, criminal" from present participle of sum, esse"to be, that which is." Some etymologists believe the Germanic word was an early borrowing directly from the Latin genitive. Also see sooth. Sin-eater is attested from 1680s. To live in sin "cohabit without marriage" is from 1838; used earlier in a more general sense. Ice hockey slang sin bin "penalty box" is attested from 1950.sin (v.) Old English syngian "to commit sin, transgress, err," from synn (see sin (n.)); the form influenced by the noun. Compare Old Saxon sundion, Old Frisian sendigia, Middle Dutch sondighen, Dutch zondigen, Old High German sunteon, German sündigen "to sin." Form altered from Middle English sunigen by influence of the noun.