affiance
Concise Oxford English Dictionary © 2008 Oxford University Press:
affiance /əˈfʌɪəns/
▶verb (be affianced) literary be engaged to marry.
– origin C15: from OFr. afiancer, from afier ‘promise, entrust’, from med. L. affidare ‘declare on oath’.
'affiance' also found in these Oxford entries:

