Stiff things don’t bend or move easily. Dead bodies and tree branches are often stiff, and your back might feel stiff if you sit for too long. But to stiff the wait staff is to skip the tip. Bad idea.
If a doorknob is stiff, it won't turn smoothly, and if the smile on your face is stiff, it’s frozen and unnatural looking. When a person is stiff, they are artificial and unrelaxed — unless they're "scared stiff" or "bored stiff." A regular guy is a working stiff. Another colloquial meaning of stiff is "to underpay, or to cheat.” Also, a stiff drink has a lot of alcohol. A dead body is sometimes called a stiff, but it’s rude.