What does "culverkey" mean?

Culverkey: An English meadow plant, perhaps the columbine or the bluebell squill (Scilla nutans). [Obs.] A girl cropping culverkeys and cowslips to make garlands. Walton. CULVER'S PHYSIC; CULVER'S ROOT Cul"ver's phys"ic, or Cul"ver's root`. [So called after a Dr. Culver, who used it.] (Bot.) The root of a handsome erect herb (Leptandra, syn. Veronica, Virginica) common in most moist woods of North America , used as an active cathartic and emetic; also, the plant itself.