When Tom Sawyer wanted to pass as a girl, he puts on a calico dress, but is found out by how he catches a lump of lead thrown in his lap:

And, mind you, when a girl tries to catch anything in her lap she throws her knees apart; she don’t clap them together, the way you did when you catched the lump of lead.

Dresses were longer then.  Today girls cross their legs, in short dresses, and men do man-spreading, even in shorts.