What is the origin of the idiom "a straight face" and when to use it?

The origin of the idiom "a straight face" can be traced back to the early 19th century. It emerged from the literal meaning of the words, where "straight" referred to something that is not crooked or bent, and "face" denoted the front part of the head. This expression gained popularity in English-speaking cultures as a way to describe the absence of a noticeable facial expression, particularly in situations where one might expect to see signs of emotion like amusement or amusement. It is used when individuals need to maintain composure or hide their true feelings, often in instances involving humor, irony, or situations that might elicit amusement.