Source : Webster's Revised Unabridged Dictionary (1913)

Obligatorily \Ob"li*ga*to*ri*ly\, adv.
   In an obligatory manner; by reason of obligation. --Foxe.

Source : WordNet®

obligatorily
     adv 1: in an obligatory manner; "this rule applies obligatorily"
            [ant: {optionally}]
     2: in a manner that cannot be evaded; "the ministry considers
        that contributions to such a fund should be met from
        voluntary donations rather than from rates compulsorily
        levied." [syn: {compulsorily}, {mandatorily}]