An extremely rare or possibly spurious term potentially relating to a group of seven; may be a variant or corrupted form of hebdomad.
Possibly a rare variant or erroneous transcription of 'hebdomad,' though documented usage is extremely limited or non-existent in standard references.
Some words exist in dictionaries more as historical curiosities than living language—'hebdomcad' might be a scribe's error that got copied so many times it seemed real, a ghost word haunting lexicons.
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