to salt: to treat, season, or preserve something with salt
Salt did three jobs in the ancient world that are worth keeping in mind at once: it flavored food, it kept meat and fish from rotting, and it sealed agreements. This verb names the simple act of applying salt, and its plain meaning was never in doubt. It is a rare, somewhat technical word, at home in treatises on cooking and farming rather than everyday speech.
Both New Testament writers who use it could have reached for the ordinary word for seasoning food (Luke does exactly that in his version of the salt saying, using ἀρτύω). Instead they chose this word, the same one the Greek Old Testament uses for salting every sacrifice laid on the altar (Leviticus 2:13, "every offering shall be salted with salt"). That choice pulls the image toward the altar and toward the "covenant of salt," where salt stood for a bond meant to last.
The two places it appears sit at very different levels. In Matthew it is literal: salt that has gone flat cannot be salted again, a warning about becoming useless. In Mark it turns figurative and famously dark, "everyone will be salted with fire," where salting and fire together suggest some process of testing, purifying, or preserving that people undergo. What the word means is settled; what that second image points to is one of the most argued questions in the Gospels.
Nuances
What "everyone will be salted with fire" (Mark 9:49) means. The lexical sense of the verb is agreed, but its figurative force is not, and the surrounding sayings about Gehenna, the divine passive form (God as the unnamed one who does the salting), and the echo of the salted sacrifices of Leviticus 2:13 pull in different directions. No reading commands a consensus.
- contestedA sacrifice image: disciples are salted as the altar offerings were, dedicated and preserved for God, echoing the salted sacrifices of Leviticus 2:13.; R. T. France, The Gospel of Mark (NIGTC; Eerdmans, 2002), 381-384; J. R. Edwards, The Gospel According to Mark (Pillar; Eerdmans, 2002), 296; J. Marcus, Mark 8-16 (Anchor Yale Bible 27A; Yale, 2009), 698-701
- contestedAn end-time testing that every person must pass through, a standalone saying about universal judgment or trial.; C. E. B. Cranfield, The Gospel According to St Mark (Cambridge Greek Testament Commentary; CUP, 1959), 315-316; M. D. Hooker, The Gospel According to Saint Mark (BNTC; Hendrickson, 1991), 230-233; A. Y. Collins, Mark: A Commentary (Hermeneia; Fortress, 2007), on 9:49
- contestedThe fiery trials of Christian life: believers are seasoned and refined through persecution and hardship, in language close to 1 Peter 1:7 and 4:12.; W. L. Lane, The Gospel of Mark (NICNT; Eerdmans, 1974), 349
- minorityA gathering rather than a salting: the verse uses the sound-alike Greek verb meaning "to gather, assemble," so that "everyone will be gathered by fire," a picture of end-time sifting.; R. Doran, "'Salting with Fire' (Mark 9:49)," Novum Testamentum 62 (2020): 361-374