As Darby and Chick watched, out of the evening came a giant beefsteak, soaring in an arc, slowly rotating, and hit the dirt almost exactly between Pugnax's front paws, where he regarded it for a while, a single eyebrow raised, one would have to say, disdainfully.
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"beefsteak"
I thought the definition of beefsteak would be more interesting than "a thick cut of beef" but that's pretty much it. It's really the same definition as steak so I don't know why there are two words for it. Was there a time in our country's history when somebody would offer you a steak and you'd accept and then they'd give you a steak from a crocodile's ass? So people had to start specifying they want "beefsteak" so that they wouldn't be tricked into eating a crocodile's butthole?
Judging by Pugnax's reaction to the beefsteak, he probably would have preferred the croc's butthole.
"As Darby and Chick watched"
This is code, like in a British crossword puzzle (or cryptic crossword as they're known in America), that means "the following sentence should be viewed as a scene from a movie." See the following note!
"soaring in an arc, slowly rotating"
This is one of those scenes where Pynchon thinks he's directing a movie. I also picture Pugnax looking at the steak and then looking directly into the camera to make his disdainful expression. Then the audience roars with laughter even though this scene was prominently featured in the trailer and they'd all already seen it. Maybe the laughter was more of the "Oh yeah! I remember seeing this in the trailer! Remember seeing this in the trailer, other movie goers also laughing uproariously?! Oh ho ho! So good, that first time I saw this. But better now because I get to experience it again while also experiencing the memory of the first time I laughed at it!"