"Like any broad social category, a single definition is guaranteed to be both too vague and too precise. Why don't you start?"
They'll say something about vaginas or ovaries, and then they're immediately on the defensive. Point out the obvious gaps they don't feel comfortable facing - women with reproductive issues, XY women, women who have had an ovarectomy, etc. Eventually they'll get to something that either resembles a purely social category, and therefore includes trans women, or an extremely long and unwieldy biological category, which you can rightly point out doesn't provides any much useful information about the actual gender role or reflect the social elements of "womanhood" they actually care about. Basically, don't define woman. Force them to do it, whittle at their arguments, and finish by saying something like "the pursuit of definitions is a waste of time on situations like this/that has no moral or cultural value".
When arguing with a reactionary, never let them set the terms for the debate. Redirect the discussion instead of playing their games.