An archaic term meaning 'henceforth' or 'from then on'; immediately afterward.
Combination of 'forth' and 'on,' likely influenced by Old English 'forthān.' The meaning shifted from literal 'forward-on' to temporal sense of 'continuing from this point.'
This word is almost extinct—it appears in Middle English texts but by Shakespeare's time had largely been replaced by 'henceforth'—yet it shows how English built temporal adverbs from spatial concepts!
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