From WordNet (r) 3.0 (2006) [wn]:
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]