"Cornetto legs" describe an unfortunate female leg shape. They are very fat/wide at the hip/thigh and taper alarmingly to tiny ankles and feet. They are not 'cankles', but are without a dainty knee as such. If the lady in question is wearing fishnet stockings, then the image is complete.
FYI: Cornettos are a cheap yet delicious ice-cream sold in the UK. They are basically conical in shape, tapering to a point, which is filled with chocolate.
Cheryl Cole has it all. A gay boyfriend, a $3 million contract for some lame US TV show and hideous Cornetto legs.