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Hi

A certain guy was swimming in the bay, then he came back home "slipped into the tiny shower with rusted piping that only delivered a thin stream of lukewarm water, like a fountain on its dying spurt. Still, he scrubbed hard, wiping away the stink and briny clutch of the angry bay - wiping away evidence, actually. He was on auto now, his mind too numb to lead the way. That would change. The head games were about to start. He could already envision the boots coming for him."

What do you think "head games" mean here? His imagination became very active? Is there a synonym for this phrase?
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The paragraph you quote gives no clue. The explanation must be farther back in context.

He seems to be a free man, in spite of his poor plumbing. Who will be wearing the boots?

I understand head games to be a sort of psychological warfare. Someone is attempting to break someone else down emotionally, or to gain the upper hand in some way through psychological manipulation. Police interrogation could be an example.

Edit. I suppose you could be right about the active imagination. There must be something in prior context to make it clear.
Has he been delusional in the past? Does he imagine people harassing him in some way? - people with boots?
If this is all in his mind, I suppose you could describe it as mind games. But that's not the usual concept. It's something one person does to another.
Hi Avangi
Avangi Police interrogation could be an example.

I think you're right about it.

The guy (Stone) was escaping from someone. He came back home and took a shower.

Already dog-tired, he once more pulled off his clothes then slipped into the tiny shower with rusted piping that only delivered a thin stream of lukewarm water, like a fountain on its dying spurt. Still, he scrubbed hard, wiping away the stink and briny clutch of the angry bay - wiping away evidence, actually. He was on auto now, his mind too numb to lead the way. That would change. The head games were about to start. He could already envision the boots coming for him.

Stone was listening for the knock on the door; it came as he was dressing.

"Hey man, you ready?" called the voice. It shot through the thin plywood door like a cat's paw into a mouse hole.


......................

They (Stone and that other guy) were loading an old banged-up, bald-tire Ford F-150 with firewood when the police car and black sedans slid into the driveway, propelling gravel in all directions like fired BBs. The trim, muscled men who climbed out of the rides wore blue slickers with "FBI" stenciled on the back in gold lettering and pistols with fourteen-round clips in their belt holsters. Three of them walked up to Stone and his buddy, while a chubby uniformed sheriff with polished black boots and a Stetson hustled to catch up.

Then the policemen started asking them questions, like: Where were you yesterday? Show me your ID etc.

Now it seems to me that "head games" means something like "questioning/interrogation". So the guy under the shower knew that the questioning would start soon and he could envision the cops (boots) coming for him.

Maybe it's some slang expression: boots = cops?
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Strange. I don't find it in Urban Dictionary, but I'd swearEmotion: thinking its a common expression for those guys.

"Boots on the ground" has been used a lot lately, but that's military.
AvangiStrange. I don't find it in Urban Dictionary, but I'd swear its a common expression for those guys.

Hi

But which expression "boots"? Emotion: smile
Just, "the boots."
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Ok, thanks Avangi!