Source : WordNet®

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}, {doze off}, {drowse off}]
        [ant: {wake up}]
     3: retreat [syn: {fall back}, {lose}, {fall behind}, {recede}]
        [ant: {gain}]
     4: get worse; "My grades are slipping" [syn: {slip}, {drop away},
         {fall away}]