Delve

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Old English delfan ‘dig’, of West Germanic origin; related to Dutch delven .

wiktionary

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From Middle English delven, from Old English delfan(“to dig, dig out, burrow, bury”), from Proto-Germanic *delbaną(“to dig”), from Proto-Indo-European *dʰelbʰ-(“to dig”). Cognate with West Frisian dolle(“to dig, delve”), Dutch delven(“to dig, delve”), Low German dölven(“to dig, delve”), dialectal German delben, telben(“to dig, delve”).

From Middle English delve, delf, dælf, from Old English delf, ġedelf(“digging”) and dælf(“that which is dug out, delf, ditch”). More at delf.

etymonline

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delve (v.)

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.