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Wiktionary
kilt pin

n. A pin, somewhat similar in shape, though larger than, a safety pin or bobby pin, which is inserted through the outer, front apron, or through both aprons of a kilt or a kilt-skirt.

Wikipedia
Kilt pin

The kilt pin is a piece of jewellery that is usually worn on the lower corner of the outer apron of a kilt. Its function is to prevent the apron falling or blowing open, by adding weight to the outer apron and is commonly seen as a form of decoration. It does not pin the outer apron to the inner fabric, which is a common misconception.

It is common for kilt pins to be decorative with many featuring clan symbols, national emblems or native animals and plants. Contemporary kilt pins often symbolise personal interests, favourite places or are personalised for the wearer.