squirrel + -ly, 1925. Presumably from erratic movement of squirrels, or by analogy with nuts (“insane”).
squirrelly (comparative squirrellier or more squirrelly, superlative squirrelliest or most squirrelly)
- resembling a squirrel
- unpredictable or jumpy
- "He is acting squirrelly."
- ^ “squirrelly” in Douglas Harper, Online Etymology Dictionary (2001).