what does 'presume away' mean? [closed]

This is distinctly idiomatic.

away = "continuously or repeatedly, or actively".

Cambridge dictionary

This definition is relevant but in my experience there are significant other implications. The Protagonist is saying something on the lines of:

"Presume as much as you like, as long as you like, until you don't want to do it any more, but all that matters to me is that you take me to Mr Crosby." He may also also be implying that the tone of the steward (by using the word presume rather than asking the simpler "Are you joining Mr Crosby at lunch, sir?") is inappropriately lofty and patronizing.

This answer is clearly ironic, and the irony might be directed at the pretentious style in which the stewart addresses the protagonist. (But it may also be related to the character of the protagonist: one can like to be ironic when one is in a good mood, or one may like to manifest a "I don't care what you think" attitude). I do not know the movie, so sorry if I misunderstand your context. But it seems to me that the protagonist picks up particularly on the verb presume. Presume away in this situation is a very laconic way of saying:

Go ahead / Suit yourself and presume to your fill! /Be my guest!