The side corner of the mouth where the upper and lower lips meet.
example
Als sie die gute Nachricht hörte, zogen sich ihre Mundwinkel zu einem Lächeln nach oben.
When she heard the good news, the corners of her mouth lifted into a smile.
Usage tips
Compound word
The word is a simple compound of 'der Mund' (the mouth) and 'der Winkel' (corner, angle). So it literally describes the corner of the mouth.
EXAMPLE
Er hatte nach dem Essen noch einen Krümel am Mundwinkel.
After eating, he still had a crumb at the corner of his mouth.
Describing feelings
'Mundwinkel' (corner of the mouth) is very often used to express feelings. When the corners of the mouth 'nach oben ziehen' (turn upward), someone is smiling. When they 'nach unten hängen' (hang down) or 'nach unten zeigen' (point downward), someone is sad or unhappy.
EXAMPLE
Als er das Kompliment hörte, zogen sich seine Mundwinkel nach oben.
When he heard the compliment, the corners of his mouth turned upward.
Typical collocations
A common and somewhat poetic expression is 'ein Lächeln umspielt die Mundwinkel' (a smile plays around the corners of the mouth). This describes a slight, often unconscious smile.
EXAMPLE
Ein zufriedenes Lächeln umspielte ihre Mundwinkel, als sie das Bild betrachtete.
A satisfied smile played around the corners of her mouth as she looked at the picture.