This literary device refers to the practice of drawing attention to a fact that is already obvious and noticeable. Understating a fact is usually done by way of sarcasm, irony, wryness or any other form of dry humor. Understating something is akin to exaggerating its obviousness as a means of humor.
The phrase, “Oh! I wonder if he could get any later; I am free all day long”. Said in a sarcastic tone it indicates that the speaker obviously means the opposite of the literal meaning.