drop off in WordNet English dictionary

1. leave or unload
   "unload the cargo"; "drop off the passengers at the hotel"
   drop  drop off  set down  put down  unload  discharge 
   
2. change from a waking to a sleeping state
   "he always falls asleep during lectures"
   fall asleep  dope off  flake out  drift off  nod off  drop off  doze off  drowse off 
   
3. get worse
   "My grades are slipping"
   slip  drop off  drop away  fall away 
   
4. fall or diminish
   "The number of students in this course dropped off after the first test"
   drop off 
   
5. retreat
   fall back  lose  drop off  fall behind  recede 
   

drop off in English Wiktionary