Meaning: (Colloquial) Something truly excellent, one of the very best of its kind.
Notes: Here is another common and popular word that you probably wouldn't want to use on your college application but is OK in almost all other circumstances. It is a lexical orphan, with no clearly related family since it is not derived from some verb humding. No, it is unrelated to that humdinger of a vehicle, the Humvee, also called a Hummer. Humvee comes from the abbreviation HMMWV of that so, so romantic name "High Mobility Multipurpose Wheeled Vehicle".
In Play: Anything that is an outstanding representative of its kind is, colloquially speaking, a humdinger: "Matt Tremoni is about to marry a humdinger of a woman with a six-figure income and a father who owns a liquor store." Well, this turns out to be a humdinger of a lie: Matt is engaged to the town librarian whose dad is in rehab.
Word History: Today a ding is the sound of metal striking metal or the dent left by metal striking metal. However, this word began its life meaning to hit, strike, or even smash. A dinger, then, would mean someone or something striking, even smashing—a hit, so to speak. Where the hum comes from remains a mystery. English does have an interjection that was once used for emphasis or to indicate deep thought. It is often spelled hmmm, but the Oxford English Dictionary lists it as hum. Might it have been added as a verbal exclamation point? (Today we thank Colin Burt for suggesting such a humdinger of a Good Word as today's