Swertainly is a modified form of the common word "certainly." It is most often used to affirm a statement someone just made. Swertainly is also used on the streets as a more endearing term of acknowledgment. When used on the streets, the phrase "my dear" is frequently added on to the end, creating: "Swertainly, my dear."
Bethany: You want to settle this in a game of ping-pong, before I kick your ass?
Mark: Swertainly, my dear.