The Collaborative International Dictionary of English v.0.48:

Capsize \Cap*size"\, v. t. & i. [imp. & p. p. Capsized; p. pr.
   & vb. n. Capsizing.] [Cf. Sp. cabecear to nod, pitch,
   capuzar, chapuzar, to sink (a vessel) by the head; both fr.
   L. caput head.]
   To upset or overturn, as a vessel or other body.
   [1913 Webster]

         But what if carrying sail capsize the boat? --Byron.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Capsize \Cap"size`\, n.
   An upset or overturn.
   [1913 Webster]

WordNet (r) 3.0 (2006):

capsize
    v 1: overturn accidentally; "Don't rock the boat or it will
         capsize!" [syn: capsize, turtle, turn turtle]