If ignorance, you should take a physics class or two. Momentum, mass, all that good stuff still plays a role in space, and greatly affects propulsion.

If sarcasm. . .if you're getting on his case because he said weight (obviously requiring a gravitational field) rather than mass. . .well. Nitpick much? You knew what he meant. Or perhaps you didn't, in which case refer to section one of this message.