When you throw your hat over the wall, you're committing to doing something. The phrase is derived from what you would do before you climb over a wall - you throw the hat over to the other side, so it doesn't fall off while you are climbing.
"throw your hat over the wall" example:

JFK himself. When he was in office, he stood before the world and promised them a man on the moon within 10 years. Thing is, nobody had started working on a space program at that point. JFK had no data to back up his claims, no insight into the practicality of space travel. But you know what he had?

The man had sac. The man had the sac to stand before the world and say “Yo, yo get this! We’re going to the moon.”
by Moggraider March 3, 2009
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To trap oneself into producing a result or achieving something monumental by making the consequences of not doing so painful, as in throwing one's hat over a wall that appears too high to climb so that one must find a way to the other side else live hatless.
"If you really want to move to that town, quit waiting for perfect circumstances; throw your hat over the wall and put in your 30-days' notice on your apartment so you'll HAVE to find a place down there."
by thatsthething March 20, 2012
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