leb-
“lip, to lick”lip, hanging skin
Root for lip, yielding Latin labium, Old English lippa.
Discussion
The PIE root *leb- (lip, that which hangs) produced the word for lip across several branches with remarkable phonological and semantic stability. The root may be connected to the broader PIE vocabulary of hanging and drooping (*leb- in its verbal sense), though whether the lip-word is primary (the lip as a body part) or derived (the lip as "the hanging thing") is debated.
Latin labium (lip) and the variant labrum (lip, edge, rim) are the central reflexes, generating a rich English vocabulary: labial (pertaining to the lips, used in both phonetics and anatomy), labiodental (a speech sound formed with lip and teeth, such as /f/ and /v/), and the combining form labio- in scientific terminology. The Latin plural labia entered English as an anatomical term for lip-shaped structures.
The Germanic branch shows the root in Old English lippa (lip), from PGmc *lepjō or *lipjō. German Lippe, Dutch lip, and the Scandinavian forms all continue the same word. The phonological variation between Latin lab- and Germanic lip- reflects the expected ablaut alternation between e-grade and i-grade (or a different reconstruction *lep- with Germanic umlaut).
Old Irish lab (lip) and Welsh llafar (utterance, voice — from the lip as organ of speech) provide Celtic attestation. The Welsh semantic development from "lip" to "speech" is a natural metonymy: the lip stands for the act of speaking, as in English lip service and stiff upper lip.
The connection to lapping and licking is semantically natural though etymologically uncertain. English lap (to lick up liquid, as a cat does) may be from a related root, and the Latin lambere (to lick) shows a similar phonological shape. If all these forms connect to a single PIE root, the semantic field encompasses both the body part (lip) and its primary functions (licking, lapping, speaking).
The word's resistance to borrowing is characteristic of basic body-part terms. English uses the native Germanic lip rather than any Latin or French derivative — unlike face (French), torso (Italian), or abdomen (Latin). Body-part vocabulary occupies one of the most conservative strata in any language, and lip has remained essentially unchanged from its Germanic ancestor for well over a thousand years.
Notes
Pokorny 655. Possibly related to leb- (to hang loosely).