WordNet (r) 3.0 (2006):

drop off
    v 1: fall or diminish; "The number of students in this course
         dropped off after the first test"
    2: change from a waking to a sleeping state; "he always falls
       asleep during lectures" [syn: fall asleep, dope off,
       flake out, drift off, nod off, drop off, doze off,
       drowse off] [ant: arouse, awake, awaken, come
       alive, wake, wake up, waken]
    3: leave or unload; "unload the cargo"; "drop off the passengers
       at the hotel" [syn: drop, drop off, set down, put
       down, unload, discharge]
    4: retreat [syn: fall back, lose, drop off, fall behind,
       recede] [ant: advance, gain, gain ground, get
       ahead, make headway, pull ahead, win]
    5: get worse; "My grades are slipping" [syn: slip, drop off,
       drop away, fall away]

Moby Thesaurus II by Grady Ward, 1.0:

88 Moby Thesaurus words for "drop off":
   abate, ablate, bate, be eaten away, buy it, cascade, cash in,
   cataract, check in, check out, collapse, come down, consume,
   consume away, corrode, crash, croak, crumble, decline, decrease,
   deliquesce, descend, die away, diminish, dip down, dive, dope off,
   down, doze off, drop, drop asleep, drop down, drowse off, dwindle,
   ebb, erode, fall, fall asleep, fall away, fall down, fall off,
   go down, go downhill, go to sleep, go west, gravitate, have it,
   kick in, kick off, kick the bucket, knock off, languish, lessen,
   let up, lose altitude, melt away, nod off, parachute, pass out,
   peg out, pipe off, pitch, plummet, plunge, pop off, pounce,
   pour down, precipitate, rain, run low, sag, settle to sleep,
   shove off, shrink, sink, slide, slump, step off, stoop, subside,
   swoop, tail off, trend downward, wane, waste, waste away, wear,
   wear away