Source : Webster's Revised Unabridged Dictionary (1913)

Oppressive \Op*press"ive\, a. [Cf. F. oppressif.]
   1. Unreasonably burdensome; unjustly severe, rigorous, or
      harsh; as, oppressive taxes; oppressive exactions of
      service; an oppressive game law. --Macaulay.

   2. Using oppression; tyrannical; as, oppressive authority or
      commands.

   3. Heavy; overpowering; hard to be borne; as, oppressive
      grief or woe.

            To ease the soul of one oppressive weight. --Pope.
      -- {Op*press"ive*ly}, adv. -- {Op*press"ive*ness}, n.

Source : WordNet®

oppressively
     adv : in a heavy and oppressive way; "it was oppressively hot in
           the office"