The Collaborative International Dictionary of English v.0.48:

Jejune \Je*june"\, a. [L. jejunus fasting, hungry, dry, barren,
   scanty; of unknown origin.]
   [1913 Webster]
   1. Lacking matter; empty; void of substance.
      [1913 Webster]

   2. Void of interest; barren; meager; dry; as, a jejune
      narrative.

   3. Juvenile; childish; immature.
      [PJC]

   4. Lacking nutritional value.
      [PJC] - Je*june"ly, adv. -- Je*june"ness, n. --Bacon.
      [1913 Webster]

WordNet (r) 3.0 (2006):

jejune
    adj 1: lacking in nutritive value; "the jejune diets of the very
           poor" [syn: insubstantial, jejune]
    2: displaying or suggesting a lack of maturity; "adolescent
       insecurity"; "jejune responses to our problems"; "their
       behavior was juvenile"; "puerile jokes" [syn: adolescent,
       jejune, juvenile, puerile]
    3: lacking interest or significance or impact; "an insipid
       personality"; "jejune novel" [syn: insipid, jejune]

Moby Thesaurus II by Grady Ward, 1.0:

208 Moby Thesaurus words for "jejune":
   Lenten, Spartan, abstemious, acarpous, ankle-deep, arid, ascetic,
   attenuated, austere, banal, barren, blah, bland, blank, bloodless,
   cadaverous, celibate, characterless, childless, cold, colorless,
   corpselike, cursory, dead, depthless, desert, desolate, dilute,
   diluted, dismal, draggy, drained, drearisome, dreary, dried-up,
   dry, dryasdust, dull, dusty, dwarfed, dwarfish, effete,
   elephantine, emacerated, emaciate, emaciated, empty, empty-headed,
   empty-minded, empty-pated, empty-skulled, epidermal, etiolated,
   exhausted, exiguous, fade, fallow, flat, flavorless, frugal,
   fruitless, gaunt, gelded, gruelly, haggard, heavy, ho-hum, hollow,
   hollow-eyed, impotent, impoverished, inane, indifferent,
   ineffectual, inexcitable, infecund, infertile, innocuous, insipid,
   issueless, knee-deep, leached, leaden, lean, lifeless, light,
   limited, low-spirited, marantic, marasmic, meager, mean,
   menopausal, mild, milk-and-water, miserly, namby-pamby, narrow,
   niggardly, nonfertile, nonproducing, nonproductive, nonprolific,
   not deep, on the surface, pale, pallid, paltry, pappy,
   parsimonious, peaked, peaky, pedestrian, pinched, plodding,
   pointless, poky, ponderous, poor, pulpy, puny, rattlebrained,
   rattleheaded, sapless, savorless, scant, scanty, scatterbrained,
   scrawny, scrimp, scrimpy, shallow, shallow-rooted, shoal,
   shriveled, sine prole, skeletal, skimp, skimpy, skin-deep, slender,
   slight, slim, slow, small, solemn, spare, sparing, spiceless,
   spiritless, stale, starvation, starved, starveling, sterile, stiff,
   stingy, stinted, stodgy, straitened, stuffy, stunted, subsistence,
   sucked dry, superficial, surface, tabetic, tabid, tasteless,
   tedious, teemless, tenuous, thin, trivial, uncultivated, underfed,
   undernourished, unfertile, unflavored, unfruitful, unlively,
   unnourishing, unnutritious, unplowed, unproductive, unprofound,
   unprolific, unsavory, unsown, untilled, vacant, vacuous, vapid,
   virgin, washy, waste, wasted, watered, watered-down, watery, weak,
   weazeny, wishy-washy, withered, without issue, wizened, wooden,
   wraithlike