The Collaborative International Dictionary of English v.0.48:

Manful \Man"ful\, a.
   Showing manliness, or manly spirit; hence, brave, courageous,
   resolute, noble. " Manful hardiness." --Chaucer. --
   Man"ful*ly, adv. -- Man"ful*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

manfully
    adv 1: in a manful manner; with qualities thought to befit a
           man; "having said her say Peggy manfully shouldered her
           burden and prepared to break up yet another home" [syn:
           manfully, manly] [ant: unmanfully, unmanly]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "manfully":
   arduously, bec et ongles, courageously, dauntlessly, effortfully,
   gamely, hammer and tongs, hard, heart and soul, heroically,
   industriously, laboriously, like a man, mettlesomely, operosely,
   pluckily, spunkily, strenuously, the hard way, toilsomely,
   tooth and nail, with a will, with effort