Old English dropa "a drop of liquid," from Proto-Germanic *drupon (source also of Old Saxon dropo, Old Norse dropi, Dutch drop, Old High German tropfo, German Tropfen (n.)), from PIE *dhreu-. Meaning "an act of dropping" is from 1630s; of immaterial things (prices, temperatures, etc.) from mid-19c. Meaning "lozenge, hard candy" is 1723. Meaning "secret place where things can be left illicitly and picked up later" is from 1931. Drop in the bucket (late 14c.) is from Isaiah ix.15 [KJV]. At the drop of a hat "suddenly" is from 1854; drop-in "casual visit" is 1819. To get the drop on someone originally was Old West gunslinger slang (1869).
Old English dropian "to fall in drops" (see drop (n.)). Meaning "to fall vertically" is late 14c. Transitive sense "allow to fall" is mid-14c. Related: Dropped; dropping. Exclamation drop dead is from 1934; as an adjective meaning "stunning, excellent" it is first recorded 1970.