Not a standard English word in modern usage; may be an archaic or obsolete term, possibly relating to reining or controlling.
Unknown standard origin; if archaic, possibly from be- + rain or rein (to control a horse), but not documented in major historical dictionaries.
The fact that this might blend 'be-' with 'rain' or 'rein' shows how our brains try to parse unfamiliar words by breaking them into known pieces—which is exactly how language change happens.
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