Green’s Dictionary of Slang

fraggle n.

[the 1980s TV series Fraggle Rock]

(UK prison) someone who is mentally disturbed.

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[UK]Sun. Times News Rev. 12 Mar. 3: His roomies were the ‘cluckers’, ‘toppers’ [...] and ‘fraggles’ (mentally disturbed).

In compounds

fraggle flops (n.)

(UK prison) prison-issue trainers.

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[UK]G. Krauze What They Was 261: Don’t wear prison-issue trainers, mandem call them fraggle flops and don’t rate you if you rock them.