hereunto has the recorded history: An archaic, formal compound from here (Old English hēr) + unto (a Middle English blend of un- and to), meaning 'to this'. Morphologically, the word is best read through the components named in the history, with the modern spelling preserving a compound structure rather than a single unanalyzable stem. In present English, the adverb sense is "to this; unto this point or matter (archaic, formal)", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as here, hereto, unto, and thereunto. Its semantic neighborhood includes hereto, herein, and hereunder, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Old English hēr and Middle English unto.