Land him one with this,sir!

[eiris]

I got this sentence in my textbook.
When the man wants to fight against another man,his maid says:"Land him one with this,sir!"(She hands him something as a weapon.)
I don't know why the"one" is here.Could you help me?

[byron]

I got this sentence in my textbook.
When the man wants to fight against another man,his maid says:"Land him one with this,sir!"(She hands him something as a weapon.)
I don't know why the"one" is here.Could you help me?

This is just colloquial 'one' being the punch. To "land him one" means to hit him. You could also say "give him a thump" or even "clock him one" Although "clock him one" may be localised to the UK only.