What are mad props?

Mad Props are kudos. It's American slang. I don't know the exact origin or who first said "mad props", but I'm pretty sure it's derived from "mad" as in, madness, or something totally unconventional & "props" being like properties, or abilities.

Props is a shortened version of proper respect, and mad means extreme.

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The last sentence above is close to the mark. A more complete and well-supported answer (though very different than the first three sentences above) can be found here: http://www.randomhouse.com/wotd/index.pperl?date=20010213