Thighs happen. I’ve personally always found that a bit disappointing, especially when I try to find pants that fit. If you’re making a bifurcated nether-garment of any sort, and you want it to fit over your thighs, it’s helpful to have a proper measurement. This measurement is also used in many men’s short-pants throughout history (canions, slops, etc).
The Thigh measurement is taken around the thigh, about midway along the femur (thigh bone). Stand in a normal posture, with your weight evenly distributed on both feet. It’s best to have someone else take this measurement for you. If you are particularly athletic in the thigh, take the measurement with the muscles relaxed and again with the muscles tensed: record the larger of the two. If one of your thighs is particularly larger than the other due to sports or a physical issue, measure the larger.