Unwholesome (4550) (sapros from sepo = cause to decay, to putrefy, to rot away, be corrupted) describes that which is rotten, putrefying, corrupt, disgusting, perishing, rank, foul, putrid, worthless (e.g., in Mt 7:17,18 = fruit, in Mt 13:48 = fish). In secular writings sapros was used to describe spoiled fish, rotten grapes on the ground, crumbling stones. The basic meaning relates to the process of decay. Sapros is used of things unusable, unfit, bad. It describes that which is harmful due to the fact that it is corrupt and corrupting or defiling. Paul presents a picture of the repugnant (and non-edifying) nature of our old self's "old garment" of rotten speech, which like rotten fruit or fish (now that's a smell you really want to avoid), will not nourish anyone. Instead the rotten fare contaminates, sickens, smells foul and creates an unpleasant atmosphere for all who come near. Using this vivid metaphor Paul commands believers to put off speech like one would toss out rotten fruit or fish! Charles Hodge says sapros... literally means “putrid,” and then figuratively “offensive and injurious.” (Ephesians 4:17-32, 5:1-2 Commentary Online) Sapros - 8x in 6v - Matt 7:17, 18; 12:33; 13:48; Luke 6:43; Eph 4:29 Matthew 7:17 -note "So every good tree bears good fruit, but the bad (sapros) tree bears bad (poneros) fruit. 18 (note) "A good tree cannot produce bad (poneros) fruit, nor can a bad (sapros) tree produce good fruit. Matthew 12:33 "Either make the tree good and its fruit good, or make the tree bad and its fruit bad; for the tree is known by its fruit. Matthew 13:48 and when it was filled, they drew it up on the beach; and they sat down and gathered the good fish into containers, but the bad they threw away. Luke 6:43 "For there is no good tree which produces bad fruit, nor, on the other hand, a bad tree which produces good fruit. Ephesians 4:29 Let no unwholesome word proceed from your mouth, but only such a word as is good for edification according to the need of the moment, so that it will give grace to those who hear. Graham in the Exegetical Summary of Ephesians... Paul is condemning any use of the faculty of speech that is morally unhealthy [Candlish, Lloyd-Jones], that suggests impure thoughts [Calvin, Candlish, Lloyd-Jones], light views of sin, irreverence towards God, or trifles with serious things [Candlish]. Profanity and obscenity, as well as careless or light speech, profane religious and sacred concepts which results in a nullification of sacred ideas conveyed by language, which is man’s most powerful weapon for influencing the thoughts and lives of his fellows [WBC]. (Graham, G.. An Exegetical Summary of Ephesians. Dallas, TX: SIL International) John Piper comments that... The image in Paul's mind is probably one of rottenness and decay, something that is spoiled. This kind of rotten language must be taken off like the old garment. It is part of the old self of Ep 4:22 that needs to be stripped away when a person becomes a Christian. The garment of a rotten mouth must be taken off and thrown into the fire, just like the Ephesians had burned their old books on magic in Acts 19:19. (Make Your Mouth a Means of Grace) The KJV Bible Commentary.... Corrupt speech comes from a corrupt heart, and pure speech comes from a pure heart. John Eadie... The general meaning of sapros is foul, rotten, useless, though sometimes, from the idea of decay—old, obsolete, ugly, or worthless. In Mt. 7:17, 18, 12:33, and in Lk 6:43, the epithet characterizes trees and their fruit, and in the Vulgate is rendered simply malus. In Matt. 13:48, it is applied to fishes. In all these places the contrasted adjective is agathos. Locke in his paraphrase has, “no misbecoming word.”...It signifies what is noxious, offensive, or useless, and refers to language which, so far from yielding “grace” or benefit, has a tendency to corrupt the hearer. 1Co 15:33; Col 4:6-note...May there not be reference to sins already condemned? All falsehoods and equivocations; all spiteful epithets and vituperation; all envious and vengeful detraction; all phrases which form a cover for fraud and chicanery—are filthy speech, and with such language a Christian's mouth ought never to be defiled. (Ephesians 4 Commentary Online) The TDNT adds that Relating to the process of decay, sepo means “to cause to decay,” or, in the passive, “to decay,” “to rot,” and figuratively “to perish.” Sapros means “rotting,” either literally or figuratively, and the sense of “unpleasant” (even to the ears) is also possible. A person is sapros when old, and the same applies to food and drink, which may be better when sapros (e.g., ripe cheese). In general, what is sapros is “unserviceable‘’ rather than “offensive,” but the word may also mean “harmful” or “notorious” (someone’s name). (Kittel, G., Friedrich, G., & Bromiley, G. W. Theological Dictionary of the New Testament. Eerdmans) From the context a "sapros" word is any word that is not good for edifying where that effect is needed. Hence in that sense a "sapros word" is an unprofitable word (as for example "idle gossip"). Our words do not have to be “dirty” to be worthless! For one thing, rotten or corrupt speech can contaminate the thoughts of others.