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Swimwear that looks like something your Grandma/Nan/Granny would wear.
Stu: "I like that swimming costume"

Jess: "What?! It's a Grankini!"
by Mobsy April 29, 2009
A bikini worn by a grandma, or an old lady.
The act of wearing this item of clothing usually results in bad reactions from public.
Man 1: Arr got a good ole Grankini to your right.
Man 2: Man, that's rough.
by FriendsFan0208 October 17, 2008