The Collaborative International Dictionary of English v.0.48:

fragmentation grenade \fragmentation grenade\ n. (Mil.)
   A type of hand grenade designed to burst into multiple
   fragments upon detonation of the explosive charge; the
   fragments fly away at high velocity, killing or wounding
   persons nearby. Contrasted to concussion grenade. The
   common type of fragmentation grenade used by the American
   military was sometimes jocosely referred to as a pineapple
   from its reticulated surface appearance, resembling that of
   the fruit.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

Pineapple \Pine"ap`ple\, n. (Bot.)
   A tropical plant (Ananassa sativa); also, its fruit; -- so
   called from the resemblance of the latter, in shape and
   external appearance, to the cone of the pine tree. Its origin
   is unknown, though conjectured to be American.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pineapple
    n 1: a tropical American plant bearing a large fleshy edible
         fruit with a terminal tuft of stiff leaves; widely
         cultivated in the tropics [syn: pineapple, pineapple
         plant, Ananas comosus]
    2: large sweet fleshy tropical fruit with a terminal tuft of
       stiff leaves; widely cultivated [syn: pineapple, ananas]