This letter presents a lockable upper-limb exoskeleton to reduce the physical fatigue of laborers/surgeons operating with upper limbs maintaining fixed posture for long time. A novel structure design integrating the jamming and engagement mechanisms that can provide locking of flexion and adduction movements of upper arm without affecting the motion space is proposed. An air-pressure waist bag with a voice control interface is developed, giving good portability and user convenience to the exoskeleton. The system is validated in experiments, indicating a locking torque as large as 6 Nm with 3.5 bar air pressure. Locking and unlocking settling times of the jamming mechanism are 0.09 s and 0.10 s, meaning nearly no delay can be felt by users for the switching. Moreover, surface electromyography (EMG) signal tests demonstrate that the proposed exoskeleton can significantly reduce muscle activation of randomly selected subjects by 25%--73%. Compared with commercially-available passive exoskeletons, proposed system has a lower weight, much larger and adjustable locking torque owing to the specially designed jamming mechanism. The exoskeleton presents good compliance, great flexibility, and hands free human-machine interface, showing good potential for physical fatigue reduction of laborers/surgeons over long-term operations.