The Collaborative International Dictionary of English v.0.48:

Sappy \Sap"py\, a. [Compar. Sappier; superl. Sappiest.]
   [From 1st Sap.]
   [1913 Webster]
   1. Abounding with sap; full of sap; juicy; succulent.
      [1913 Webster]

   2. Hence, young, not firm; weak, feeble.
      [1913 Webster]

            When he had passed this weak and sappy age.
                                                  --Hayward.
      [1913 Webster]

   3. Weak in intellect. [Low]
      [1913 Webster]

   4. (Bot.) Abounding in sap; resembling, or consisting largely
      of, sapwood.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sappy \Sap"py\, a. [Written also sapy.] [Cf. L. sapere to
   taste.]
   Musty; tainted. [Obs.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

sappy
    adj 1: ludicrous, foolish; "gave me a cockamamie reason for not
           going"; "wore a goofy hat"; "a silly idea"; "some wacky
           plan for selling more books" [syn: cockamamie,
           cockamamy, goofy, sappy, silly, wacky,
           whacky, zany]
    2: abounding in sap; "sappy maple trees"; "sappy kindling wood"

Moby Thesaurus II by Grady Ward, 1.0:

121 Moby Thesaurus words for "sappy":
   absurd, apish, asinine, balmy, bathetic, batty, beery, befooled,
   beguiled, besotted, brainless, budding, buffoonish, callow,
   cloying, cockeyed, crazy, credulous, daffy, daft, dazed, dewy,
   dizzy, doting, drippy, dumb, fantastic, fatuitous, fatuous, flaky,
   flowing, fluent, fluid, fluidal, fluidic, fluxible, fluxile,
   fluxional, fluxionary, fond, fool, foolheaded, foolish, fuddled,
   futile, gaga, gooey, goofy, green, growing, gulled, gushing,
   harebrained, idiotic, imbecile, immature, impubic, inane, inept,
   inexperienced, infatuated, ingenuous, innocent, insane, intact,
   juicy, kooky, liquid, liquidy, loony, mad, maudlin, mawkish, minor,
   moronic, mushy, naive, namby-pamby, new-fledged, nostalgic,
   nostomanic, nutty, oversentimental, oversentimentalized,
   preposterous, raw, ripening, romantic, runny, screwy, senseless,
   sentimental, sentimentalized, silly, sloppy, slushy, soft, soppy,
   soupy, sticky, stupid, succulent, tear-jerking, teary, tender,
   thoughtless, unadult, underage, undeveloped, unfledged, unformed,
   unlicked, unmellowed, unripe, unseasoned, vernal, virginal, wacky,
   watery, wet, witless