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
- ^ “squirrelly”, in Douglas Harper, Online Etymology Dictionary, 2001–.