das Kinn
chin
The lowest part of the human face, below the mouth.
Nachdenklich stützte er sein Kinn in die Hand.
Thoughtfully, he rested his chin in his hand.

chin
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chin
The lowest part of the human face, below the mouth.
Nachdenklich stützte er sein Kinn in die Hand.
Thoughtfully, he rested his chin in his hand.

Be careful: 'das Kinn' means 'chin' in English. It is a false friend of the English word 'kin', which means 'Verwandtschaft' (relatives/family).
Er hat ein sehr markantes Kinn.
He has a very prominent chin.
Common actions connected with the chin are 'das Kinn auf die Hand stützen' (to rest one's chin on one's hand), 'sich am Kinn kratzen' (to scratch one's chin), or 'das Kinn heben' (to lift one's chin).
Sie stützte nachdenklich ihr Kinn auf die Hände.
She thoughtfully rested her chin on her hands.
'Kinn' (chin) is part of other useful words that refer to this part of the face, for example 'das Doppelkinn' (double chin) or 'der Kinnbart' (goatee/chin beard).
Er trägt einen gepflegten Kinnbart.
He has a well-groomed goatee.
jaw
'der Kiefer' and 'das Kinn' both belong to the lower area of the face and are therefore easy to confuse. 'Der Kiefer' is the jawbone in which the teeth sit. For example, people talk about the 'Oberkiefer' (upper jaw) or 'Unterkiefer' (lower jaw). 'Das Kinn' is the visible part at the very bottom of the face, under the mouth. If the topic is teeth, biting, or pain while chewing, 'Kiefer' usually fits. If you mean the appearance of the face, 'Kinn' is more likely to fit.

Beim Sturz hat sich Tom das Kinn an der Tischkante gestoßen.
When he fell, Tom hit his chin on the edge of the table.
Nach dem Essen tut ihr der Kiefer weh, wenn sie den Mund weit öffnet.
After eating, her jaw hurts when she opens her mouth wide.