Definition: Your emotions are clearly shown in your expression
Example: Jim’s disappointment was written all over his face. He had wanted that first edition Star Wars comic so badly.
Iddy could not find any origin theories for this idiom. He supposes the reasons behind it are fairly evident. Your expression can be read as clearly as the written word.
Iddy hasn’t written just any old thing on his face for this demonstration. Extra smug points to you if you can identify the classic novel opening page he has transcribed on his own green skin.