President Obama: Props to the K.C. Royals ... But Your Nicknames Suck!

The K.C. Royals got some real talk from President Obama Thursday -- who gave 'em all sorts of props for winning the World Series ... but told 'em their nicknaming skills are pretty weak.

The Royals got to visit 1600 Pennsylvania Ave. as personal guests of POTUS -- and Obama did his usual thing ... roasting the team with some friendly zingers.

Among the topics ... nicknames, hashtags and Fetty Wap.