If you've ever watched a young child have a tantrum in a store because their parents won't buy them something they want, you know exactly what sniveling means. To call someone sniveling is a real insult — you're implying that they're worse than childish. The adjective comes from snivel, "cry and sniffle" or "whine," from the Old English snyflan, "to run at the nose," from the root snofl, "nasal mucus."