What the h6?

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I-m_Thinking-OK
I have been playing 1.e4 as white recently and after 1...e5 2.Nf3 d6 3.Bc4 they play 3...h6. They play this immediately, so I'm guessing its some philidor system, but it doesn't seem to make sense. Also, the opening explorer does not have a name for it, so I can not even see if its some YouTuber opening or something. Anyone know what opening this is? Thank you.
Uhohspaghettio1

It's the 11th (!) most popular move there in the database and not in the top 5 computer lines.

I'm guessing it's just clueless people parroting moves they've seen good players make or read about and without any opening theory. They might think they're stopping Ng5 with it.

User49578
 
Keep playing the Italian Game, but wait with 0-0, because maybe the idea of h3 is g2-g4, with the potential g4-g5 and a kingside attack.
 
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If black plays 0-0, your DSB (dark-squared bishop) can play Bxh6. His DSB on the other hand (which is likely to go to e7) will block his queen from getting to the queenside.
 
You can see that his pieces are too cluttered under these pawns, so their chance to get into good squares is small.
tygxc

@1

They fear 3...Nf6 4 Ng5, though 4...d5 5 exd5 Nxd5 6 Nxf7 Kxf7 7 Qf3+ Ke6 8 Nc3 c6 is playable.
It makes no sense: 3...h6 loses a tempo and weakens.
Just 4 O-O Be7 5 d4 gives white an advantage.