Old English hlanc "loose and empty, meagerly slim, flaccid," from Proto-Germanic *hlanka-, forming words meaning "to bend, turn," perhaps from PIE root *kleng- "to bend, turn," with a connecting notion of "flexible" (compare German lenken "to bend, turn aside;" see flank (n.)). In Middle English, "Some examples may be long adj. with unvoicing of g" [The Middle English Compendium]. In reference to hair, "straight and flat," from 1680s. Related: Lankness (1640s).
The noun is not found in Old English, where it is represented by lank "the hip" ("turn of the body"), hlencan (plural) "armor." Meaning "a division of a sausage made in a continuous chain" is from mid-15c. Meaning "anything serving to connect one thing or part with another" is from 1540s. Sense of "means of telecommunication between two points" is from 1911. Missing link between man and apes dates to 1880.
also rat's-tail, from rat (n.) + tail (n.1). Used since 16c. of conditions, growths, or devices held to resemble a rat's long, hairless tail in any way, including "lank lock of hair" (1810); "end of a rope" (1867). Related: Rat-tailed. A rat-tail file (1744) is a fine, round file used for enlarging holes in metal.
late 14c., twiteren, in reference to birds, of imitative origin (compare Old High German zwizziron, German zwitschern, Danish kvidre, Old Swedish kvitra). The noun meaning "condition of tremulous excitement" is attested from 1670s. The microblogging service with the 140-character limit was introduced in 2006. The following is considered an unrelated word of obscure origin:
TWITTER. 1. "That part of a thread that is spun too small." Yarn is said to be twined to twitters, when twined too small, S. Hence, to twitter yarn, to spin it unequally, A. Bor. Ray.
2. It is transferred to any person or thing that is slender or feeble. It is said of a lank delicate girl: "She is a mere twitter," S. [Jamieson, "Etymological Dictionary of the Scottish Language," Edinburgh, 1808]