late 14c., "that which is dropped," verbal noun from drop (v.). Specifically "dung" (especially of fowls) from 1590s. Related: Droppings.
Middle English droppen, from Old English dropian "to fall in drops, fall in small portions or globules, as a liquid." The word is part of a related series of verbs in Proto-Germanic that also yielded Old Saxon driopan, Old Frisian driapa, Dutch druipen, Old High German triufan, German triefen, and in English drip, droop, and obsolete dreep and dripe. Related: Dropped; dropping.
In reference to a solid object, "to fall vertically" from late 14c. The transitive sense "allow to fall" is from mid-14c. To drop in "visit casually" is from c. 1600; drop-in (n.) "a casual visit" is attested by 1819. The notion in drop (someone) a line "write a letter" (1769) is of dropping a message into a letter-box. Exclamation drop dead to express emphatic dislike or scorn is from 1934; as an adjective meaning "stunning, excellent" it is recorded by 1970 (compare killing, etc.).
"the letter H," representing the pronunciation of the letter-name, by 1887, originally especially in reference to dropping it in colloquial speech.
"to fall or fall into with a sound like 'plop,' " 1821, imitative of the sound of a smooth object dropping into water. Related: Plopped; plopping. Thackary (mid-19c.) used plap (v.). As a noun from 1833.
1540s, of animals, "process of bringing forth young in a single birth," verbal noun from present participle of litter (v.). Meaning "act of furnishing with bedding" is from c. 1600. That of "act of dropping disordered waste matter" is from 1900.