The Collaborative International Dictionary of English v.0.48:

Weakly \Weak"ly\, adv.
   In a weak manner; with little strength or vigor; feebly.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Weakly \Weak"ly\, a. [Compar. Weaklier; superl. Weakliest.]
   Not strong of constitution; infirm; feeble; as, a weakly
   woman; a man of a weakly constitution.
   [1913 Webster]

WordNet (r) 3.0 (2006):

weakly
    adv 1: in a weak or feeble manner or to a minor degree; "weakly
           agreed to a compromise"; "wheezed weakly"; "he was weakly
           attracted to her" [ant: strongly]
    adj 1: lacking bodily or muscular strength or vitality; "a
           feeble old woman"; "her body looked sapless" [syn:
           decrepit, debile, feeble, infirm, rickety,
           sapless, weak, weakly]

Moby Thesaurus II by Grady Ward, 1.0:

125 Moby Thesaurus words for "weakly":
   a bit, a la sourdine, a little, anemic, asthenic, barely,
   bloodless, by a hair, by an ace, cachectic, chicken, cowardly,
   daintily, debilitated, decrepit, delicately, dimly, drained,
   drooping, droopy, dull, effeminately, effete, enervated, etiolated,
   ever so little, exhausted, exiguously, failing, faint, faintish,
   faintly, feeble, feebly, flabby, flaccid, flimsy, floppy, fragile,
   frail, gently, gone, gutless, hardly, healthless, hushedly,
   imbecile, imperfectly, impotent, in poor health, inappreciably,
   inconsequentially, infirm, infirmly, insignificantly,
   insubstantial, invalid, just a bit, languid, languishing,
   languorous, languorously, lightly, limber, limp, listless,
   listlessly, little, low, lustless, marrowless, meagerly, minimally,
   minutely, moribund, negligibly, nerveless, not hardly, only just,
   pale, peaked, peaky, pianissimo, piano, pithless, pooped,
   powerless, reduced, reduced in health, rubbery, run-down, sapless,
   scantily, scarcely, shakily, sickly, sinewless, slack, slightly,
   soft, softly, sordamente, sordo, spineless, strengthless,
   strengthlessly, subduedly, tant soit peu, teeteringly, totteringly,
   triflingly, unhardened, unhealthy, unnerved, unsound, unsoundly,
   unsteadily, unstrung, unsubstantial, unsubstantially,
   valetudinarian, valetudinary, weak, weakened,
   with low resistance