The joint design of origami-inspired robots is one of the most distinctive features that distinguishes them from conventional robots. A joint design using material's compliance enables origami robots to implement complex transformational movements in a lightweight and simple manner. However, utilizing the continuum bending mode of materials brings critical problems, including undesired movements and joint radius. This study introduces a solution to these problems through a torsion-based compliant joint (T-C joint) design, which utilizes the torsion deformation of materials. The potential of the T-C joint is demonstrated in a flat-foldable and self-assembling robotic arm, providing its applicability in environments with form-factor limitations and minimal human intervention. The robotic arm - comprising links, joints, and a gripper - can fold into a flat state, deploy with precision and minimal weight, and effectively manipulate target objects. This demonstration shows the real-world application of the proposed joint design.