The Raspberryemote - the action is often referred to as "blowing a raspberry" - is a default emote which is unlocked upon creation of a character. Any player can use it. The emote is used as a way to disrespect or playfully tease another player.
If a player blew a raspberry at Muncher, Death's skeleton dog, during the 2009 Hallowe'en event, he would kill them instantly. However, they would respawn in the main reception hall of the mansion without losing any items. Because of the "no risk" factor, many players had fun blowing raspberries at Muncher.
When shooing away a stray dog in Varrock or Meiyerditch, a player will perform this emote and shout "Thbbbbt!", causing the dog to whine and run away. Once this is performed, the "Thbbbbt!" will appear in the chatbox.