indorse has the recorded history: A variant of endorse, from Medieval Latin indorsare, from Latin in- ('on') + dorsum ('back'); literally 'to write on the back of'. The path is chiefly one of transmission: the word passes through the named languages and forms, with each stage preserving part of the modern shape. In present English, the verb sense is "to endorse; to sign the back of a document or give approval", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as endorse and dorsal, and in derivatives such as indorsement, indorser, and indorsee. Its semantic neighborhood includes endorse, sign, countersign, and approve, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Latin in- and Latin dorsum.