Snappity Dapity is a phrase used to express your excitement in someone'e elses behaviour. It is an extension of the phrase "oh, snap!". It may be used to emphasize someone else getting burned, a congratulations, a way of telling a person you are impressed with something they said.
John is watching a t.v. show and the main character makes a move on an attractive woman. John smoothly says, "snappity dappity" to express his excitement for the main character.