In Monterey before he even started, he felt hungry and stopped at Herman’s for a hamburger and beer. While he ate his sandwich and sipped his beer, a bit of conversation came back to him. Blaisedell, the poet, had said to him, “You love beer so much. I’ll bet some day you’ll go in and order a beer milk shake.” It was a simple piece of foolery but it had bothered Doc ever since. He wondered what a beer milk shake would taste like. The idea gagged him a bit but he couldn’t let it alone. It cropped up every time he had a glass of beer. Would it curdle like milk? Would you add sugar? It was like a shrimp ice cream. Once the thing got into your head you couldn’t forget it. He finished his sandwich and paid Herman. He purposely didn’t look at the milk shake machines lined up so shiny against the back wall. If a man ordered a beer milk shake, he thought, he’d better do it in a town where he wasn’t known. But then, a man with a beard, ordering a beer milk shake in a town where he wasn’t known — they might call the police.

So let’s be clear. In the 40s, the thought of a beer milkshake could warrant police activity. Later in the chapter, Doc goes ahead and takes the plunge and discovers what a beer milkshake tastes like:

Doc walked angrily to the counter of the stand.
The waitress, a blond beauty with just the hint of a goiter, smiled at him. “What’ll it be?”
“Beer milk shake,” said Doc.
“What?”
Well here it was and what the hell. Might just as well get it over with now as some time later.
The blond asked, “Are you kidding?”
Doc knew wearily that he couldn’t explain, couldn’t tell the truth. “I’ve got a bladder complaint,” he said. “Bipalychaetorsonechtomy the doctors call it. I’m supposed to drink a beer milk shake. Doctor’s orders.”
The blonde smiled reassuringly. “Oh! I thought you were kidding,” she said archly. “You tell me how to make it. I didn’t know you was sick.”
“Very sick,” said Doc, “and due to be sicker. Put in some milk, and add half a bottle of beer. Give me the other half in a glass — no sugar in the milk shake.” When she served it, he tasted it wryly. And it wasn’t so bad — it just tasted like stale beer and milk.
“It sounds awful,” said the blonde.
“It’s not so bad when you get used to it,” said Doc. “I’ve been drinking it for seventeen years.”

I read this Steinbeck passage as a teenager. I wonder if it had anything to do with my proclivity for beer margaritas.