duty bound, to be

To be firmly obligated. This term is derived from bounden duty, which dates from the 1500s and was actually redundant, since from the 1400s bound also meant “under obligations.” Nevertheless, it appears in the Communion Service of the Book of Common Prayer (1559): “We beseech thee to accept this our bounden duty and service.” It also retains this form three centuries later: “It was his bounden duty to accept the office” (Harriet Martineau, The Manchester Strike, 1833). At some point this locution was grammatically changed to the present participial usage, as in “I’m duty bound to report this violation to the dean.”