The Collaborative International Dictionary of English v.0.48:

Exacting \Ex*act"ing\, a.
   Oppressive or unreasonably severe in making demands or
   requiring the exact fulfillment of obligations; harsh;
   severe. "A temper so exacting." --T. Arnold --
   Ex*act"ing*ly, adv. -- Ex*act"ing*ness, n.
   [1913 Webster]