Interferometry, a typical phase-sensitive measurement, usually requires additional hardware and complex signal processing to achieve subwavelength resolution. In this paper, we propose a potential thickness variation monitor, in which only reflection losses are needed to achieve deep-subwavelength accuracy. The proposed monitor is based on a sensitive thickness-dependent metamaterial absorber (MMA), which is made of split copper rings periodically arranged in a flexible and nonabsorbent thermoplastic polyurethane base. Experimental results showed that the frequency of the absorption peak increases almost linearly in the range from 6.655 to 6.755 GHz when the thickness of the MMA decreases from 25.625 to 24.833 mm with a step of 0.0417 mm, about a thousandth of the central wavelength of this frequency region. The effects of two key geometry parameters, i.e., the distance from the split copper ring to the metal base and the gap width of the split copper ring, on the absorption performance are investigated by numerical simulation. Our results may provide a simple method of monitoring deep-subwavelength thickness variation in the far field.