The kilt is a garment, not a costume ...
You can just tell them to take a peak at a dictionary and educate themselves ...

According to Oxford American Dictionary

kilt |kilt|
noun
a knee-length skirt of pleated tartan cloth, traditionally worn by men as part of Scottish Highland dress and now also worn by women and girls.
kilt
verb [ trans. ]
gather (a garment or material) in vertical pleats : [as adj. ] ( kilted) kilted skirts.
DERIVATIVES
kilted adjective
ORIGIN Middle English (as a verb in the sense [tuck up around the body] ): of Scandinavian origin; compare with Danish kilte (op) ‘tuck (up)’ and Old Norse kilting ‘a skirt.’ The noun dates from the mid 18th cent.