What does Boxing Day have to do with boxing?
mid-15c. (implied in smelter), from Dutch or Low German smelten, from Proto-Germanic *smelt- (cf. Old High German smelzan, German schmelzen "to melt"), from PIE *smeld-, variant of *mel- "soft." Thus the word is from a variant of the stem of Old English meltan "to melt" (see melt (v.)). Related: Smelted; smelting.
Old English smelt "sardine, small salmon-like sea fish," cognate with Dutch smelt "sand eel," Danish smelt (c.1600). OED notes that it has a peculiar odor (but doesn't suggest a connection with smell); Klein suggests a connection with the way the fish melts in one's mouth. Century Dictionary speculates it means "smooth" and compares Old English smeolt, smylt "serene, smooth."