Lava® soap, the gritty bar for heavy-duty hand cleaning, is well named. The grit is pumice, a form of volcanic rock or lava. And volcanic “lava” like the “lava-“ in “lavatory” comes from the Latin lavāre ‘to wash.’
Lave, an Old English word meaning ‘to wash or bathe’ is now used only in poetry. Shakespeare employed it a few times with the alternate spelling “laue”: “Wee must laue Our Honors in these flattering streames.” Macbeth (1623) iii. ii. 34.
The Oxford English Dictionary gives the etymology: < Latin lavāre = Greek λούειν, < Old Aryan root *lou- to wash (whence lather n.).
< Latin lavātōrium a place for washing, < lavāre to wash: see lave v.1
From the 14th century “lavatory” was used to mean ‘a vessel for washing, a laver or bath.’ By the 17th century it meant a room for hand washing and eventually a room that also included a water-closet or toilet. In the 20th century it was a common British expression for the room known in Canada as a “washroom” and in the U.S. as a “restroom” in a public place and a “bathroom” in a private home (whether or not it contains a bath).
Lava [from the OED]< Italian lava ( < lavare to wash: see lave v.1), originally ‘a streame or gutter suddainly caused by raine’ (Florio 1611), applied in the Neapolitan dialect to a lava-stream from Vesuvius; hence adopted in literary Italian, where it developed the senses represented by 2 and 3 below. 1. A stream of molten rock issuing from the crater of a volcano or from fissures in the earth. 2. The fluid or semi-fluid matter flowing from a volcano. 3. The substance that results from the cooling of the molten rock.
On the Big Island of Hawaii a huge lava flow once swept through a forest leaving giant lava pillars where the lava cooled and hardened around tree trunks. British visitors are often confused or amused by the name of park containing the ghostly sentinels: Lava Tree State Monument. Those Yanks must be daft. Why would they name a park after a W.C.?