"Shit or get off the pot" is a common English language colloquial expression, used to imply a person should follow up their stated intentions with action. It is also used to urge someone to complete a task with a greater degree of efficiency or timeliness than is observed at the time the expression is used. Implicit in the expression is that the person, by failing to act, is preventing others from acting.
Origin
It may derive from the time before indoor plumbing when a chamber pot was used for defecation (though the modern toilet continues to be referred to as "the pot" sometimes). The expression, in this way, is essentially instruction for someone to stop being indecisive or procrastinating, and act. Stuart Flexner, author of "I Hear America Talking", states that it is a vulgar rephrasing of the old New England expression "Fish or cut bait".