The Addendum to "The Shorter English Dictionary on Historical Principles"
gives the etymology of 'escapee', meaning 'One who has escaped' as "[f.
ESCAPE v. + -EE after Fr, echappe, subst. use of pa. pple. of echapper.]".
Sorry I can't do the italics or the acute accent on the second e of echappe.
accepted 'by virtue of usage'.