WordNet (r) 3.0 (2006):

pin down
    v 1: define clearly; "I cannot narrow down the rules for this
         game" [syn: pin down, peg down, nail down, narrow
         down, narrow, specify]
    2: place in a confining or embarrassing position; "He was
       trapped in a difficult situation" [syn: trap, pin down]
    3: attach with or as if with a pin; "pin up a picture" [syn:
       pin down, pin up]

Moby Thesaurus II by Grady Ward, 1.0:

87 Moby Thesaurus words for "pin down":
   allocate, anchor, assign, balance, ballast, bind, bridle, chain,
   collocate, counterbalance, denominate, deploy, designate,
   determine, dispose, emplace, enchain, entrammel, fasten, fetter,
   firm, firm up, fix, freeze, get a fix, gyve, hamper, handcuff,
   hobble, hog-tie, hold, home in on, hopple, immobilize, indicate,
   install, keep, lash, leash, localize, locate, make fast, manacle,
   mark, mention, moor, nail down, name, navigate, peg down, pick out,
   picket, pinion, pinpoint, place, point out, position, put in irons,
   put in place, restrain, retain, rope, secure, select, set, shackle,
   signify, situate, specialize, specify, spot, stabilitate,
   stabilize, state, steady, stick, stipulate, straitjacket, strap,
   tether, tie, tie down, tie up, trammel, transfix, triangulate,
   zero in on