The Greek adjective halykos (ἁλυκός) means salty or brackish, relating to the nature of salt. It appears in James's vivid question about the impossibility of a spring producing both fresh and salt water at the same time — used to illustrate the inconsistency of blessing and cursing from the same mouth.
James 3:12 uses the impossibility of salt water producing fresh water as an analogy for double-tongued hypocrisy. The mouth that both blesses God and curses people is as contradictory as a spring producing both kinds of water. Integrity demands a unified source. The transformed heart — renewed by the Spirit — produces speech that is consistently pure, like fresh water from a good spring (Matthew 12:35).