The Collaborative International Dictionary of English v.0.48:

Under-arm \Un"der-arm\, a. (Cricket)
   1. Done (as bowling) with the arm not raised above the elbow,
      that is, not swung far out from the body; underhand. Cf.
      Over-arm and Round-Arm.
      [1913 Webster]

   2. For use under the arms, specifically in the armpit; as, an
      underarm deodorant.
      [PJC]

WordNet (r) 3.0 (2006):

underarm
    adv 1: with the hand swung below shoulder level; "throwing a
           ball underarm" [syn: underarm, underhand]
    adj 1: with hand brought forward and up from below shoulder
           level; "an underhand pitch"; "an underhand stroke" [syn:
           underhand, underhanded, underarm] [ant: overarm,
           overhand, overhanded]