Grandmaster Flash -- Stupid Valet Gave My Car ... to a THIEF!!!

Grandmaster Flash lost his prized and customized possession because an overly generous -- and not so bright valet -- simply handed over the keys to a car thief.

Flash tells us his custom made 2014 white Dodge Charger-- nicknamed The Ghost -- was jacked from a parking structure in NYC last Thursday. Most frustrating of all ... he says the parking attendant didn't ask for any ID or parking ticket from the thief.

Law enforcement sources tell us cops were called, but that was merely to take down a stolen car report. The Ghost had long since vanished by the time cops got there.

Flash -- famous for rap classics like "White Lines" and "The Message" -- also lost some vintage vinyl records that were in his whip. He's considering a lawsuit against the parking company.

Don't push him 'cause he's close to the edge ...