The Collaborative International Dictionary of English v.0.48:

Slay \Slay\, v. t. [imp. Slew; p. p. Slain; p. pr. & vb. n.
   Slaying.] [OE. slan, sl?n, sleen, slee, AS. sle['a]n to
   strike, beat, slay; akin to OFries. sl[=a], D. slaan, OS. &
   OHG. slahan, G. schlagen, Icel. sl[=a], Dan. slaae, Sw. sl?,
   Goth. slahan; perhaps akin to L. lacerare to tear to pieces,
   Gr. ????, E. lacerate. Cf. Slaughter, Sledge a hammer,
   Sley.]
   To put to death with a weapon, or by violence; hence, to
   kill; to put an end to; to destroy.
   [1913 Webster]

         With this sword then will I slay you both. --Chaucer.
   [1913 Webster]

         I will slay the last of them with the sword. --Amos ix.
                                                  1.
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         I'll slay more gazers than the basilisk. --Shak.
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   Syn: To kill; murder; slaughter; butcher.
        [1913 Webster]

WordNet (r) 3.0 (2006):

slay
    v 1: kill intentionally and with premeditation; "The mafia boss
         ordered his enemies murdered" [syn: murder, slay,
         hit, dispatch, bump off, off, polish off,
         remove]

Moby Thesaurus II by Grady Ward, 1.0:

87 Moby Thesaurus words for "slay":
   amuse, annihilate, assassinate, becharm, beguile, bereave of life,
   bewitch, bump off, butcher, captivate, carry away, carry off,
   charm, chloroform, convulse, cool, cut down, cut off, delectate,
   delight, deprive of life, destroy, dispatch, dispose of, divert,
   do away with, do for, do in, do to death, down, dust off, enchant,
   end, enliven, enrapture, enravish, entertain, enthrall, entrance,
   execute, exhilarate, exterminate, fascinate, finish, finish off,
   fracture one, freak out, immolate, imparadise, kill, knock dead,
   knock off, knock out, launch into eternity, lay low, liquidate,
   loosen up, lynch, make away with, martyr, martyrize, poison, purge,
   put away, put down, put to death, put to sleep, raise a laugh,
   raise a smile, ravish, recreate, refresh, regale, relax,
   remove from life, sacrifice, send, solace, starve, take life,
   take off, thrill, tickle, tickle pink, titillate, transport, wow