Quiet possibly the manliest method of drinking a can of beer ever. To frumble, one shakes a beer can until the carbonation builds up inside until the sides of the can become hard. Then one proceeds to hit the center of the can's side against one's forehead until the pressure causes the can to slightly rupture, spraying one's forehead with a fine misting of beer. Immediately one bites into the side of the can near the bottom while holding it upright, pops the top, and proceeds to drink the beer through the hole in its side as if one was shotgunning
Because he was lacking in manliness (and car keys), the boat engineer decided to frumble a beer rather than just plain shotgunning
it in an attempt to prove himself to his coworkers.