I've seen this expression probably a dozen times in anime and manga. There are many variants, but they all have the character's tongue sticking up out of the side of their mouth. Their eyes are also typically big and sometimes in the shape of a backwards の character, or sometimes winking. Here are a few examples, though there are many more:
Tonari no Kashiwagi-san:
What is the origin of these expressions? Are they referring to a particular art style or character?