What was Big Willy Shakes going for?
How cute, right? Well, you might be surprised to know that back in Shakespeare's day, "thing" was often used to as a euphemism for genitalia. Now this line isn't looking so innocent anymore, is it? Shakespeare certainly loved him some wordplay, especially when he could make an inappropriate joke out of it.
We will say though that we think Rosalind's male costume in this scene allows her to be bolder than usual. Sure, Rosalind is never one to shy away from speaking her mind, but she is also bound by the customs of the day to some extent. Why else would she dress like a man on the road? Rosalind seems to have keyed into a very forward-thinking idea for her day. She knows that gender is a social role that can imitated. That's why it's so easy for her to pretend to be a guy—all she has to do is act macho, and people will assume she's a man, no problem.
Even more progressively, it is Rosalind who takes charge in her romantic relationship with Orlando; scripting them, and deciding that they will marry. So it is no surprise that she is the one to make lewd jokes at their mock nuptials as well.