Is it weird for man to shave his armpits?

not at all many men manscape and include their pits

The prophet Muhammad instructed his followers to shave their armpits and their pubics. So it isn't weird for Muslim men. 

Also many athletes are self conscious about showing their pits and shave them. 

Yes and no. There is a weird social stigma against male grooming, or at least the discussion of male grooming. People agree that unkempt armpits are disgusting, but men get uncomfortable talking about things like shaving their armpits publicly. Hell, even trimming nose-hair is something reserved for "old guys" to talk about...

Personally, what I do is shave my armpits once during the summer. The only people who notice are those I wind up sleeping with, because I wear t-shirts and sweaters and **** like that. By that point, either they don't care...or they understand why someone would shave one of the sweatiest parts of the body on one of the hottest parts of the year.

Not at all. Especially if you do sports or yoga. Nobody wants to see that under arm fluff.

Many trim if they have excessive hair there . . . . most find shaving makes things worse as it's a bad place to have stubble.

No I would if I could I don't like having it

No at all. Men shave most of their body nowadays

Bikers, swimmers, and only one else, to cut down air and/or Water resistance.

Different, yes. Weird? If it makes you comfortable NOT to have hairy armpits, do it.