A direct attraction towards a person with knobbly knees.
This attraction can be further enhanced by a persons ‘slender’ or ‘chicken legs’, generally achieved by ‘skipping leg day’
Wife with knobbly knee fetish - “Wow, your knobbly knees look lovely. I will have to order three drinks to cool me down”
Husband - “thanks luv, I always skip leg day to make them even better for you”