The Collaborative International Dictionary of English v.0.48:

Boyish \Boy"ish\, a.
   Resembling a boy in a manners or opinions; belonging to a
   boy; childish; trifling; puerile.
   [1913 Webster]

         A boyish, odd conceit.                   --Baillie.
   [1913 Webster]

WordNet (r) 3.0 (2006):

boyish
    adj 1: befitting or characteristic of a young boy; "a boyish
           grin"; "schoolboyish pranks" [syn: boyish, boylike,
           schoolboyish]

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "boyish":
   adolescent, airy, attenuate, attenuated, beardless, boylike,
   calflike, childish, childlike, coltish, delicate, diaphanous,
   diluted, ethereal, fine, fine-drawn, finespun, flimsy, frail,
   gauzy, girlish, girllike, gossamer, gracile, immature,
   insubstantial, juvenile, kiddish, lacy, light, maiden, maidenly,
   misty, papery, puerile, puplike, puppyish, puppylike, rare,
   rarefied, slender, slenderish, slight, slight-made, slim, slimmish,
   slinky, small, subtle, svelte, sylphlike, tenuous, thin,
   thin-bodied, thin-set, thin-spun, thinnish, threadlike, vague,
   wasp-waisted, watered, watered-down, watery, weak, willowy,
   wiredrawn, wispy, young, youthful