Christina Milian -- who's sporting a big piece of ice on her finger -- has been playing coy about whether or not she's actually engaged ... but it kinda ruins the game when she calls her man ... her "fiancé."
Christina was at Supperclub in Hollywood last night -- flashing her huge diamond ring -- when we tried the oldest trick in the book ... we congratulated her on her engagement.
Watch ... she almost fell for it.
By the way, Rap-A-Lot records CEO Jas Prince is the alleged purchaser of Christina's alleged engagement ring. Allegedly.