Flipyap (achievement)
From Halopedia, the Halo wiki
Flipyap is an achievement in Halo: The Master Chief Collection. It is unlocked by listening to a Grunt talk about Flipyap and his brother, Yapflip. The achievement is worth 5 Gamerscore.
In order for the achievement to unlock, the player must trigger the quote. It is not necessary for the quote to finish, or even be heard by the player; as soon as the sound file is triggered, the achievement will unlock. The quote will only be triggered with the IWHBYD skull turned on, and most often occurs when the player kills a Grunt while another is nearby to say the quote. Even then, the quote is triggered randomly. As such, some players may unlock the achievement very quickly, while others are forced to wait a frustratingly long time.