When an extremely obese person's thigh fat is so out of hand that it appears to engulf the knee, bypass the shin and calf and ends up stopping at the ankles. Combination of "thigh" and "ankles".
Brett: Hey, Did you see that woman standing in the buffet line?
Thankles are when you are massively fat and you surpass cankles ( when your so fat you cant see where there ankle fat begins and the calf fat ends) making it impossible to see where your thigh fat ends and your ankle fat begins.
1: For when the thigh extends to the foot, without the slightest of tapering.
2: A more severe case of Cankles.
Bobby: " Dude, did you see those girls? They were hot! I'd hit that!"
Fuzz: "Nah man. Did you see the girl in the middle? That girl had thankles!!!"
Group: "Thankles?"