lip: the fleshy rim of the mouth, and by extension the speech and the language it produces
The first and physical sense is the lip itself, the soft edge of the mouth. Because the lips are where words are formed, the Hebrew slides easily from the body part to what it does: what a person says, the character of their talk, and finally the whole tongue or language a people speaks.
This is why one word covers a wide span in English. In the wisdom writings it is constantly the lips as the source of speech: lips that lie or speak truth, that flatter or instruct, that praise God. The lips become the visible surface where the hidden heart shows itself, which is why so many proverbs weigh what the lips do. Pushed one step further the word names a whole language, as when all the earth had a single "lip" before Babel, or when Isaiah speaks of a people of an unintelligible "lip." This last leap, to a whole language, is rarer than English translations suggest: it appears in only a handful of passages, nearly always about foreign or hard-to-understand speech, and Hebrew's usual word for a language is a different one, "tongue." At Babel in particular, where English says "one language," the Hebrew says "one lip," and scholars disagree over whether that means a single tongue, a shared common language used alongside local ones, or a picture of the forced sameness an empire imposes.
The thread holding these together is plain: the lips are the visible instrument of speech, so naming the lip can mean naming the words, the manner of speaking, or the tongue itself. The reader rarely needs to choose, because the verse makes clear whether the body part, the talk, or the language is in view.
Nuances
What "one lip" (usually translated "one language") means at Babel, given that the previous chapter has already divided humanity into many tongues, and that Hebrew's ordinary word for a language is not this word but "tongue."
- majorityIt means one language in the plain sense: a single universal speech that God then multiplied, whether the point is to explain where the world's languages came from or to mock Babylon.; Hermann Gunkel, Genesis, HKAT (1901; ET 1997); Gordon J. Wenham, Genesis 1-15, WBC 1 (1987), 234-246; Nahum M. Sarna, Genesis, JPS Torah Commentary (1989), 80-83; Robert Alter, The Five Books of Moses (2004), 58-60
- minorityIt means a shared common language, a trade or diplomatic tongue that existed alongside the local languages of the previous chapter, not humanity's only speech.; Victor P. Hamilton, The Book of Genesis: Chapters 1-17, NICOT (1990), 350; John H. Walton, Genesis, NIV Application Commentary (2001), 366-381
- minorityThe word still means "language" in the dictionary, but the whole phrase ("one lip and unified words") is a loaded political and religious expression modeled on the Neo-Assyrian royal boast of making conquered peoples "of one mouth"; on this reading Babel attacks an empire's drive to impose sameness and is not about a lost original language.; Christoph Uehlinger, Weltreich und "eine Rede", OBO 101 (1990); Samuel L. Boyd, "Sargon's Dur-Sharrukin Cylinder Inscription and Language Ideology," JNES 78 (2019): 87-111; Andre LaCocque, The Captivity of Innocence: Babel and the Yahwist (2010)