A dialectal or archaic term, possibly referring to a basic or fundamental principle, or potentially related to groundsill/grundsil (a foundation timber).
Possibly from German Grund (ground, foundation) with the -el suffix, or related to grundsil/groundsill. The etymology is uncertain, but it appears in some English dialects and older texts.
Grundel is so obscure that its meaning has largely been lost—it's a ghost word that reminds us that English dialects once had rich vocabularies that mostly disappeared as standard English spread.
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