coltish

adjective3

1. •
   Resembling a colt.
2. •
   Lively, playful and undisciplined (often in a manner judged to be immature).

   • Plants rais’d with tenderness are seldom strong, Man’s coltish disposition asks the thong [i.e. whip], And without discipline the fav’rite child, Like a neglected forrester runs wild. — 1782
3. •
   Tall, thin and awkward (especially of an older child or adolescent).

   • He had a tangle of shock hair, the colour of wool; his mouth was a grin; although as strong as a horse, he looked neither heavy nor yet adroit, only leggy, coltish, and in the road. — 1883