The Collaborative International Dictionary of English v.0.48:

Busy \Bus"y\ (b[i^]z"z[y^]), a. [OE. busi, bisi, AS. bysig; akin
   to D. bezig, LG. besig; cf. Skr. bh[=u]sh to be active,
   busy.]
   1. Engaged in some business; hard at work (either habitually
      or only for the time being); occupied with serious
      affairs; not idle nor at leisure; as, a busy merchant.
      [1913 Webster]

            Sir, my mistress sends you word
            That she is busy, and she can not come. --Shak.
      [1913 Webster]

   2. Constantly at work; diligent; active.
      [1913 Webster]

            Busy hammers closing rivets up.       --Shak.
      [1913 Webster]

            Religious motives . . . are so busy in the heart.
                                                  --Addison.
      [1913 Webster]

   3. Crowded with business or activities; -- said of places and
      times; as, a busy street.
      [1913 Webster]

            To-morrow is a busy day.              --Shak.
      [1913 Webster]

   4. Officious; meddling; foolish active.
      [1913 Webster]

            On meddling monkey, or on busy ape.   --Shak.
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   5. Careful; anxious. [Obs.] --Chaucer.
      [1913 Webster]

   Syn: Diligent; industrious; assiduous; active; occupied;
        engaged.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Busy \Bus"y\ (b[i^]z"z[y^]), v. t. [imp. & p. p. Busied
   (b[i^]z"z[i^]d); p. pr. & vb. n. Busying.] [AS. bysgian.]
   To make or keep busy; to employ; to engage or keep engaged;
   to occupy; as, to busy one's self with books.
   [1913 Webster]

         Be it thy course to busy giddy minds
         With foreign quarrels.                   --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

busy
    adj 1: actively or fully engaged or occupied; "busy with her
           work"; "a busy man"; "too busy to eat lunch" [ant:
           idle]
    2: overcrowded or cluttered with detail; "a busy painting"; "a
       fussy design" [syn: busy, fussy]
    3: intrusive in a meddling or offensive manner; "an interfering
       old woman"; "bustling about self-importantly making an
       officious nuisance of himself"; "busy about other people's
       business" [syn: interfering, meddlesome, meddling,
       officious, busy, busybodied]
    4: crowded with or characterized by much activity; "a very busy
       week"; "a busy life"; "a busy street"; "a busy seaport"
    5: (of facilities such as telephones or lavatories) unavailable
       for use by anyone else or indicating unavailability;
       (`engaged' is a British term for a busy telephone line); "her
       line is busy"; "receptionists' telephones are always
       engaged"; "the lavatory is in use"; "kept getting a busy
       signal" [syn: busy, engaged, in use(p)]
    v 1: keep busy with; "She busies herself with her butterfly
         collection" [syn: busy, occupy]

Moby Thesaurus II by Grady Ward, 1.0:

102 Moby Thesaurus words for "busy":
   absorb, active, arabesque, assiduous, at it, at work,
   attend to business, baroque, burden, bustling, busybody, chichi,
   complex, complicated, detailed, devote, diligent, divert, drive,
   elaborate, elegant, employ, employed, energetic, engage, engaged,
   engross, fag, fancy, fine, flamboyant, florid, flowery, forward,
   frilly, full of business, fussy, hard at it, hard at work, hectic,
   high-wrought, hustling, immerse, impertinent, in harness,
   industrious, inquisitive, intricate, involve, involved, keep busy,
   labored, lively, luxuriant, luxurious, meddlesome, meddling,
   mind the store, moresque, nosy, occupied, occupy, officious,
   on duty, on the go, on the hop, on the job, on the jump,
   on the move, on the run, oppress, ornate, ostentatious, overdrive,
   overelaborate, overelegant, overlabored, overtask, overtax,
   overwork, overworked, overwrought, pass the time, picturesque,
   presumptuous, pretty-pretty, prying, pushing, pushy, rich, rococo,
   self-appointed, snoopy, soak, spend, spend the time, sweat, task,
   tax, tied up, work, working