Abnormal joint rigidity is a common symptom of many neurological disorders like Parkinson's disease and stroke. Joint rigidity is clinically described as increased resistance during passive joint movement and is quantitatively related to the static passive component of joint impedance, i.e., passive stiffness. Here, we introduce a novel approach to estimate the passive stiffness of the wrist in humans across two coupled Degrees of Freedom (DoFs): Flexo-Extension (FE) and Radio-Ulnar Deviation (RUD). This method employs a subject-specific kinematic model of the human wrist, treated as a universal joint with two skew-oblique axes (FE and RUD), not intersecting and not necessarily perpendicular one to each other. We tested this methodology on ten healthy volunteers using infrared cameras and a hand-held device equipped with a 6-axis load cell for manual wrist perturbations. We used motion and force/torque data to determine angles and torques at the wrist DoFs, and applied multiple linear regression to calculate the 2-by-2 stiffness matrix. Our findings align with existing literature in terms of stiffness behavior, showing stiffness anisotropy with the highest value predominantly along the RUD direction. However, compared to a simplified wrist model with intersecting, orthogonal axes and to prior studies assuming alignment between human joints and robotic ones, our method demonstrates significant differences, particularly in the magnitude of the stiffness ellipse. By providing a more anatomically plausible representation of wrist kinematics through relaxing the constraints of simplified methods, our results show, for the first time, an accurate subject-specific estimation of wrist stiffness.