An archaic or poetic adverb meaning 'to this place' or 'hither'; calling someone or something toward the speaker.
From 'be-' (directional prefix) + 'hither' (toward here, from Old English 'hider'). The 'be-' prefix reinforced the directional sense of movement.
Old English and Middle English had tons of directional adverbs (hither, thither, whither) that we've largely lost—'behither' is an extra-emphatic version, showing how speakers once had fine-grained ways to describe direction and movement.
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