The poem opens with a question that sounds almost like a shrug: if someone is locked up, why act surprised when they behave like it? The speaker seems to be reflecting on a person, or perhaps a creature, shaped by confinement, and the title suggests that the confinement may be as much mental as physical. The phrase "mind-forged manacles" points to chains we build inside our own heads.
What the speaker describes is anger at life itself, a "wilderness of use and abuse." That image is bleak: not a free, wild place, but a landscape of damage. The last two lines add a quiet sting. This life has "no wonder," only "entertainment," as if distraction has replaced real curiosity or awe.
The mood is weary and sharp, with a hint of sympathy underneath. The speaker doesn't shout. The short lines and plain wording make the judgment feel calm, and that calm makes it land harder. Judging by the title, the "prison" may reach well beyond cell walls.