get one's ass in / into gear

What is the origin of the idiom "get one's ass in gear" and when to use it?

The idiom "get one's ass in gear" is an idiomatic expression that originated in American English, particularly in the mid-20th century. It is a colloquial and somewhat vulgar phrase that conveys the idea of urging someone to start working or taking action with greater speed, efficiency, or urgency, as if their own body or efforts were a mechanical system that needed to engage or operate more effectively. It is often used when someone is perceived as being slow, lazy, or unproductive and needs to be motivated or prompted to take action.