The term is related to but predates griefer, and was reserved for players disruptive only to their own team. A llama is different from a noob in that the player is not new to the game but in fact uses their familiarity with the game to purposefully harass their own team.
Probably one of the most well-known examples of llama behavior, crouching in front of the exit to your team's respawn in TFC used to prevent teammates from leaving the respawn room but kept the door open for enemies to shoot/spam grenades through the door, leaving your team defenseless.
Coming from the song: "Don't Wanna Trust a Llama" REMIX by Chynaman feat. Savage
You know her llama has been around the block, no... the neighborhood, no... the community! lol
Ooo I wouldn't hit that cause everyone knows she has a dirty llama