An archaic or obsolete conjunction meaning 'because' or 'for the reason that,' used in Middle English.
From 'for-' (for the reason) + 'because' (by cause). This is a fossilized remnant of Middle English where 'forbecause' and 'because' competed, with 'because' ultimately winning.
Language is full of the corpses of words that lost evolutionary battles—'forbecause' was a perfectly logical way to say 'because' but couldn't compete with the simpler version, so it died out.
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