A globule first, then almost everything else
In present-day English, drop still names the small rounded mass that a liquid forms as it falls. It also names the fall itself, a steep vertical distance, a sudden decline in price or temperature, a hard candy shaped like a bead, a theater curtain, and a hidden place where something can be left for later pickup. The verb covers the same range: rain drops, a cup drops, someone drops a subject, a visitor drops in.
That spread looks disorderly until the history is lined up. The physical globule came first. Almost every later sense is a figure of falling, of a tiny amount, or of a round hanging shape.
Native English, not a loan
The origin of the word drop is inheritance, not borrowing from French or Latin. Middle English had drope and later droppe for the noun, and droppen for the verb. Those forms continue Old English dropa, a masculine noun for a drop of liquid, and the weak verb dropian, also spelled droppian, already used of liquid falling in globules.
Both the noun and the verb are recorded in the Old English period, before 1150. The Oxford English Dictionary treats the verb as formed inside English from the noun. Similar weak verbs turn up in other early West Germanic languages, including Old High German tropfōn, but those look like independent formations on the same noun rather than one inherited verb shared across the family.
A related Old English frequentative, droppettan, meant to drip or fall in drops. It sits beside Old High German forms of the same frequentative type and shows that speakers kept building verbs off the same base. Whether the earlier English spellings with a single p reflect a real sound change or an accident of the surviving manuscripts is not settled.
The Germanic cluster
Behind Old English dropa stand Proto-West Germanic dropō and Proto-Germanic drupô, a noun for a drop of liquid. That noun is tied to a reconstructed verb dreupaną, with senses around dripping, hanging down, and falling in drops.
Cognates make the family easy to see. Old Saxon had dropo. Old Norse had dropi, continued in modern Scandinavian as Danish dråbe, Swedish droppe, Norwegian dråpe or drope, and Icelandic dropi. Dutch keeps drop and drup. Old High German tropfo became German Tropfen. Verb cousins include Old Saxon driopan, Old Frisian driapa, Dutch druipen, and German triefen.
English itself split the same cluster into several words. Drip is a close relative. Droop came in from Old Norse drūpa, to hang down. Older English also had dreep and dripe, now obsolete. The American Heritage Dictionary groups drop with this set under an Indo-European root written dhreu-, with senses of falling, flowing, dripping, and drooping, and it also places dreary on that root through Old English drēor, gore, via the idea of dripping blood. That last link is a reconstruction, not something a modern speaker would guess from the surface of drop.
Where the reconstructions disagree
Germanic ancestry is solid. The deeper Indo-European step is not. Some etymological dictionaries trace dreupaną to a root dʰrebʰ- meaning to drip or drop. American Heritage uses dhreu- with the falling and flowing senses above, and an extended zero-grade dhrub- as the immediate source of Germanic drupan. Other handbooks still list a Pokorny etymon dhreu- as to grind or crumble, and they put Old English dropa in that family as well, sometimes explaining trickle and fall as later developments from crumbling. Those reconstructions are not the same story, and none of them should be treated as a settled last word. What can be said with less hedging is the Germanic noun-and-verb nest and the path through Old English.
From liquid to fall, then to almost any small amount
By about 1200 the noun was already used for a minute quantity of anything, not only of liquid. That is the sense in a drop of pity or not a drop of evidence. The late-fourteenth-century phrase a drop in the bucket, later familiar from the King James rendering of Isaiah, leans on the same idea: a globule against a vast remainder.
The verb widened from liquids to solids. The transitive sense, to allow something to fall, is recorded from the mid-fourteenth century; the intransitive sense, a solid object falling vertically, from later in that century. Once falling was the core image, the noun could name the act of dropping, from the 1630s, and, by the mid-nineteenth century, a decline in prices, temperatures, and other measures that have no physical globule at all.
Shape, not motion, produced the candy sense: a lozenge called a drop by 1723 because it looked like a bead of liquid. Hanging ornaments, chandelier pendants, and the theatrical painted curtain lowered between scenes, attested by 1779, follow the same visual. A secret pickup point, recorded from 1931, is a place where something is left to fall into someone else’s hands.
Everyday idioms keep the old image in motion. To drop in, a casual visit, is from around 1600. To drop someone a line, from 1769, pictures a note going into a letter-box. At the drop of a hat, meaning suddenly, is from 1854. To get the drop on someone, originally American gunfighter slang from 1869, meant having the first, ready shot. Drop dead as a curse is from 1934; as slang for strikingly excellent it appears by 1970, in the same grim-to-glowing pattern as killing.
A pair you can still hear
German Tropfen and English drop are still obviously kin, even after High German shifted the initial consonant. That audible pair is a better takeaway than any single Indo-European formula: the word never left the Germanic household, and the globule of liquid is still the oldest sense sitting under all the later falls.