Genuinely tricky in tone — it can mean nervous, awkward laughter ('well, this is a disaster but I'm smiling through it') just as easily as relieved amusement, so context matters a lot. Because of that ambiguity, it became an early meme staple for self-deprecating 'I'm fine, everything is fine' captions long before anxiety-face emoji existed.
Keywords: cold, dejected, excited, face, grinning, mouth, nervous, open, smile, smiling, stress, stressed, sweat