The Collaborative International Dictionary of English v.0.48:

Lightly \Light"ly\, adv.
   1. With little weight; with little force; as, to tread
      lightly; to press lightly.
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            Yet shall thy grave with rising flowers be drest,
            And the green turf lie lightly on thy breast.
                                                  --Pope.
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            Him thus intent Ithuriel with his spear
            Touched lightly.                      --Milton.
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   2. Swiftly; nimbly; with agility.
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            So mikle was that barge, it might not lightly sail.
                                                  --R. of
                                                  Brunne.
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            Watch what thou seest and lightly bring me word.
                                                  --Tennyson.
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   3. Without deep impression.
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            The soft ideas of the cheerful note,
            Lightly received, were easily forgot. --Prior.
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   4. In a small degree; slightly; not severely.
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            At the first he lightly afflicted the land of
            Zebulun . . . and afterward did more grievously
            afflict her.                          --Is. ix. 1.
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   5. With little effort or difficulty; easily; readily.
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            That lightly come, shall lightly go.  --Old Proverb.
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            They come lightly by the malt, and need not spare
            it.                                   --Sir W.
                                                  Scott.
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   6. Without reason, or for reasons of little weight.
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            Flatter not the rich, neither do thou willingly or
            lightly appear before great personages. --Jer.
                                                  Taylor.
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   7. Commonly; usually. [Obs.] --Bp. Fisher.
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            The great thieves of a state are lightly the
            officers of the crown.                --B. Jonson.
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   8. Without dejection; cheerfully. "Seeming to bear it
      lightly." --Shak.
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   9. Without heed or care; with levity; gayly; airily.
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            Matrimony . . . is not by any to be enterprised, nor
            taken in hand, unadvisedly, lightly, or wantonly.
                                                  --Book of
                                                  Common Prayer
                                                  [Eng. Ed.].
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   10. Not chastely; wantonly. --Swift.
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WordNet (r) 3.0 (2006):

lightly
    adv 1: without good reason; "one cannot say such things lightly"
    2: with few burdens; "experienced travellers travel light" [syn:
       lightly, light]
    3: with little weight or force; "she kissed him lightly on the
       forehead" [syn: lightly, softly, gently]
    4: indulging with temperance; "we eat lightly in the morning"
       [ant: hard, heavily, intemperately]
    5: with indifference or without dejection; "he took it lightly"
    6: in a small quantity or extent; "spread the margarine thinly
       over the meat"; "apply paint lightly" [syn: thinly,
       lightly] [ant: thickly]
    7: to a slight degree; "her speech is only lightly accented"

Moby Thesaurus II by Grady Ward, 1.0:

82 Moby Thesaurus words for "lightly":
   a bit, a little, airily, arbitrarily, barely, blithely, by a hair,
   by an ace, capriciously, cheerfully, cheerily, easily,
   effortlessly, ever so little, exiguously, facilely, faintly,
   fancifully, feebly, flightily, freely, frivolously, genially,
   gladly, hands down, happily, hardly, hopefully, idly, immaterially,
   imperfectly, inadvertently, inappreciably, inconsequentially,
   insignificantly, irrepressibly, jauntily, joyfully, just a bit,
   like clockwork, like nothing, little, meagerly, minimally,
   minutely, negligently, negligibly, no sweat, not hardly,
   on easy terms, only just, optimistically, permissively, pettily,
   pleasantly, readily, remissly, scampingly, scantily, scarcely,
   shallowly, simply, skimpingly, slightingly, slightly, slurringly,
   smilingly, smoothly, superficially, swimmingly, tant soit peu,
   triflingly, trivially, unessentially, unimportantly, unreasonably,
   unrigorously, weakly, well, whimsically, with good cheer,
   without difficulty