Thirsty Grunt Achievement in Halo: The Master Chief Collection

This Easter Egg can be found in the mission, The Maw.

During the Warthog escape sequence at the end of this mission, you will eventually reach an outdoor bridge where Foe Hammer is meant to pick you up. After your pilot is shot down by Covenant Banshees, continue driving your Warthog through the tunnel just ahead. After descending a small hill inside this tunnel, there will be a narrow passageway on the right side. Drive the vehicle through this small tunnel and there will be yet another open passageway on your right.

At the end of this second tunnel, there will be a lonely Grunt standing around a few dead Flood bodies. Get out of the Warthog and approach the Grunt. After a few seconds he will repeat the following line of dialogue, “Good thing that Food Nipple's waiting for me at the starship 'cause man, whew, have I worked up a big, grunty thirst!” After the Grunt finishes his line of dialogue, the achievement will unlock.