Synonyms for Drop Off (same or very similar meaning)
WordNet sense 1 (leave or unload, especially of passengers or cargo):
discharge, drop, unload, discharge, set down, put downWordNet sense 2 (retreat):
lose, recede, fall back, fall behindWordNet sense 3 (get worse):
slip, fall away, drop awayWordNet sense 4 (change from a waking to a sleeping state):
fall asleep, dope off, doze off, drift off, drowse off, flake out, nod offFrom the ODE community, based on WordNetadd/edit