A circular-slot-inscribed square-shaped metal-free superwideband absorber is presented. It comprises a silicon dioxide substrate sandwiched between the circular-slot-loaded square-shaped and square graphite slabs. It has an overall volume of <inline-formula> <tex-math notation="LaTeX">5\times 5\times 6.77\,\,\mu \text{m} </tex-math></inline-formula> with an operating frequency spectrum ranging from 5.1 to > 80 THz (> 176.03% for <inline-formula> <tex-math notation="LaTeX">{A}\ge {90} </tex-math></inline-formula>%). The periodicity and substrate thickness are <inline-formula> <tex-math notation="LaTeX">{\sim }\lambda _{\text {o}} /{12} </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">{\sim }\lambda _{\text {o}} / {76} </tex-math></inline-formula> respectively at 5.1 THz. The proposed absorber's performance is insensitive to the polarization angle (<inline-formula> <tex-math notation="LaTeX">\Phi </tex-math></inline-formula>) variations due to symmetric geometry. In addition to this, the incidence angle (<inline-formula> <tex-math notation="LaTeX">\theta </tex-math></inline-formula>) variations between 0° and 45° also had insignificant effects on the absorption. The presented absorber has advantages of metal-free geometry, polarization angle insensitiveness to <inline-formula> <tex-math notation="LaTeX">\Phi </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">{0}^{\circ }\le \theta \le {45}^{\circ } </tex-math></inline-formula> along with compact dimensions over previously reported absorbers operating in terahertz (THz)/infrared frequency ranges. The proposed absorber-based sensor has a peak sensitivity of 9 THz/RIU for refractive index (RI) sensing and 7 THz/RIU for cancer and tuberculosis detection.