What it is:
Legging out means to unwind part of a transaction.
How it works (Example):
Let's say John Doe conducts an options straddle, which involves buying a call and a put with identical expiration dates. His broker first has to purchase the call, then purchase the put, though he does this nearly simultaneously. John decides that he wants to get out of the put transaction, so he calls his broker and says he wants to leg out of the put. The broker legs out by selling the put that John just bought.