That's a very old fashioned key... Given the axe, shield and horned helmet, maybe she's married? Married viking women would carry the key to the house in their belt. The man didn't have a key, so the wife could lock her husband out when she was angry with him.
Smile!
(The noun, not the verb. Probably a mistranslation on the artist's part as it would make more sense as an encouragement to smile.)