What do you define as a SHY guy?

it seems to me that being shy or not being shy clears up a lot of mystery when a girl tries to figure out of a guy likes him.

now tell me, what exactly defines a shy guy.

i mean, there are guys who are shy in general with everyone, quiet, don't participate, don't have a lot of friends, have mellow convos

then there are guys who are popular, talk to girls and make friends easily, hangs with his boys, goes clubbing drinking, and is even friends with madddddd female friends, but can these guys be 'shy' to the one girl he really likes?

is it a specific behavior displayed to ONLY ONE person, or a general trait in someone displayed to EVERYONE?
What do you define as a SHY guy?
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