Silly, eccentric, or acting a bit crazy; or full of loops or circular shapes.
From 'loop' (possibly from Old Norse 'hlaup' meaning leap), with the suffix '-y' meaning having the quality of. The mental illness meaning likely developed from 19th-century slang about being 'off the loop' or disconnected.
The word 'loopy' perfectly captures the double helix shape of DNA—both loopy and full of loops—which scientists also sometimes call 'twisted' just like a loopy person's thoughts!
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