What does Boxing Day have to do with boxing?
mid-15c., "tasting of salt, impregnated with salt," from salt (n.) + -y (2). Meaning "racy" is from 1866, from salt in the sense of "that which gives life or pungency" (1570s, originally of words or wit). Meaning "racy, sexy" is from 1866. U.S. slang sense of "angry, irritated" is first attested 1938 (probably from similar use with regard to sailors, "tough, aggressive," attested by 1920), especially in phrase jump salty "to unexpectedly become enraged." Related: Saltily.