BobK is correct, "already" originated as a Yiddish expression. It is used for emphasis, or to express exasperation. Sometimes we add "and be done with it" to add extra emphasis.
"I really like John, but I don't know if he likes me..."
"Why don't you just tell him how you feel already!"
"That cheesecake looks so delicious, but I really shouldn't have any...."
"Oh, just eat a piece already, and be done with it!"
