Noun
A circular cage for a squirrel or other small animal, which rotates vertically as the animal runs at the bottom.
(figuratively, by extension) A tedious, repetitive, unfulfilling activity or situation, especially one in which no progress is achieved.
(engineering) Any other device, such as a fan or rotor, that resembles a squirrel cage in form.
Source: en.wiktionary.orgNever feel self-pity, the most destructive emotion there is. How awful to caught up in the terrible squirrel cage of self. Millicent Fenwick