captured living plunder: the people and animals a victor takes alive and keeps
The word is built on the ordinary verb for taking, and it names what gets taken alive. A battle report could sweep the goods, the herds, and the prisoners together under one heading, "spoil" (שָׁלָל), but this word picks out the breathing part of the haul: the men, women, children, and animals led away rather than carried away.
Numbers 31 uses it as an accounting term. The soldiers bring back the prisoners, the living catch, and the goods; Moses is then told to take a headcount of the catch "in man and in beast", split it evenly between the men who fought and the rest of the people, and set a share aside for the LORD. Everything the word covers can be counted, fed, and handed over, because it is alive.
Isaiah takes the same word and turns it into a challenge. If a strong warrior is holding a catch, can anyone pry it loose from him? The expected answer is no, and that is the point: God says he will do it anyway, and the prisoners of the strong will be taken back. The same picture of a stronger one stripping the strong shows up later in Jesus' saying about the armed householder who is overpowered and has his goods divided (Luke 11:21-22).