A versatile, functional training device of substantially elliptical shape and comprising a fluid weight that employs principles of unstable resistance as a means of increasing balance, stability, overall strength, core strength and proprioception while increasing lean muscle mass and burning calories. During operation, a user moves the device through a regimen of individualized movements, and the fluid weight moves and shifts inside the device, creating a instability at a point of resistance. The device comprises D-shaped handles with finger grooves adapted for maneuverability. The handles also may be filled with fluid to increase the weight and adaptability of the device across multiple activities and physical regimens. Measuring scales are provided on the device to measure the amount of fluid added to or removed from the device. The device may be transparent or translucent, and the fluid weight may be tinted to aid in visibility.