'assoil' originates from Old French, specifically from verbs such as 'asoillier'/'asoillir', where the root is related to Latin 'solvere' meaning 'to loosen, release'.

'assoil' changed from Old French (e.g. 'asoillier'/'asoillir') into Middle English forms (e.g. 'assoilen') and eventually into the rare modern English verb 'assoil' and its past/p.p. 'assoiled'.

Initially it carried the sense 'to loosen or release' (in a literal or figurative sense); over time it came to be used specifically with the sense 'to free from guilt or blame' (to absolve).