Used in place of a person's name when you forget their name, but you think the person you're talking to knows who it is.
Billy- "Dad, is Mr. ... what's his face? ... taking you to the airport for your business trip, or do you need a ride?"
Dad- "Oh, Mr. Smith? No, actually I fired him this morning. Can you take me before you go to work?"
Billy- "Sure Dad, since you're the greatest!"