The Collaborative International Dictionary of English v.0.48:

Plainly \Plain"ly\, adv.
   In a plain manner; clearly.
   [1913 Webster]

WordNet (r) 3.0 (2006):

plainly
    adv 1: unmistakably (`plain' is often used informally for
           `plainly'); "the answer is obviously wrong"; "she was in
           bed and evidently in great pain"; "he was manifestly too
           important to leave off the guest list"; "it is all
           patently nonsense"; "she has apparently been living here
           for some time"; "I thought he owned the property, but
           apparently not"; "You are plainly wrong"; "he is plain
           stubborn" [syn: obviously, evidently, manifestly,
           patently, apparently, plainly, plain]
    2: in a simple manner; without extravagance or embellishment;
       "she was dressed plainly"; "they lived very simply" [syn:
       plainly, simply]

Moby Thesaurus II by Grady Ward, 1.0:

106 Moby Thesaurus words for "plainly":
   all joking aside, alone, aloud, apparently, apprehensibly,
   articulately, artlessly, audibly, barely, bluffly, bluntly,
   broadly, brusquely, candidly, casually, clearly, coherently,
   commonly, commonplacely, comprehensibly, conspicuously, definitely,
   directly, discernibly, distinctly, en famille, evidently,
   exclusively, explicitly, expressly, familiarly, frankly, genuinely,
   glaringly, guilelessly, in all conscience, in all seriousness,
   in plain English, in plain terms, in plain words, informally,
   ingenuously, intelligibly, just, limpidly, lucidly, manifestly,
   markedly, matter-of-factly, merely, naturally, noticeably,
   observably, obviously, offhand, offhandedly, only, openheartedly,
   openly, ordinarily, out, out loud, outspokenly, palpably, patently,
   pellucidly, perceivably, perceptibly, perspicuously,
   plain-spokenly, point-blank, prominently, pronouncedly,
   prosaically, purely, recognizably, relaxedly, roundly,
   sans ceremonie, seeably, simply, simply and solely, sincerely,
   singly, solely, staringly, starkly, straightforwardly,
   to the point, unaffectedly, unambiguously, unassumingly,
   unceremoniously, unconcealedly, unconstrainedly, understandably,
   undisguisedly, unequivocally, unmistakably, unofficially,
   unreservedly, unrestrainedly, visibly, with clarity,
   with no nonsense, without ceremony