Nervous, jumpy, erratic, or suspicious-acting; showing the restless, quick movements associated with squirrels.
From 'squirrel,' which comes from Greek 'skiouros' meaning 'shadow-tailed,' combined with '-ly' to make an adjective; the personality meaning emerged because squirrels dart around unpredictably.
Squirrels actually have a genius behavior called 'scatter-hoarding'—they bury thousands of nuts and intentionally forget where some are buried, which helps forests regrow, but that nutty unpredictability is exactly why 'squirrelly' means acting weird and scattered.
Complete word intelligence in one call. Free tier — 50 lookups/day.