Middle English delven, from Old English delfan "to dig, turn up with a spade or other tool, excavate" (class III strong verb; past tense dealf, past participle dolfen), common West Germanic (cognates: Old Saxon delban, Dutch delven, Middle High German telben "to dig"). This is perhaps from a PIE root *dhelbh- (source also of Lithuanian delba "crowbar," Russian dolbit', Czech dlabati, Polish dłubać "to chisel;" Russian dolotó, Czech dlato, Polish dłuto "chisel").
Weak inflections emerged 14c.-16c. Figurative sense of "carry on laborious or continued research" is from mid-15c. Related: Delved; delving; delver.