lav

The root lav comes from Latin lavāre, "to wash," with a close sibling luere, "to wash, to cleanse, to wash away." Picture a Roman at a basin, pouring water over hands and face before a meal. That single image of running water fans out into a surprisingly large family, because "washing" turns out to be about flowing liquid — and once you have flowing liquid, you can dilute it, overflow with it, pour it out, or let it carry dirt away.

The most literal branch stays close to the basin. lavatory is, etymologically, "the place where you wash" — only later did it become a polite word for the toilet. laundry and launder trace through Old French lavanderie back to lavāre: the room and the act of washing clothes. (In a 20th-century twist, launder picked up the slang sense of "wash" dirty money clean.) lave is the bare old verb "to wash," now mostly poetic — water laves the shore. ablution (ab- away + luere) is a ritual washing, the water carrying impurity away from the body.

The second branch is about liquid in excess. lavish originally meant a downpour — something poured out without restraint — and the word slid from "a flood of rain" to "spending without limit." deluge (de- down + luere) is the great washing-down: a flood. lava is the spectacular member — molten rock that flows down a volcano like a river of fire; Italian named it from the Latin idea of something washing/streaming down a slope. lotion is simply "a washing," a liquid you apply to the skin.

The third branch uses luere for mixing and fouling water. dilute (di- apart + luere) means to wash apart — to thin a liquid by adding more liquid. pollute is the dark twin: from por- (an old form of pro-, "forth") + luere, it meant to wash over and defile — to dirty water or land. So dilute and pollute are cousins built on the same "wash" root, one weakening a substance, the other contaminating it.

The pattern across the whole family: wherever water flows, lav/lu follows — to clean (laundry, ablution), to thin (dilute), to overflow (lavish, deluge), to stream down (lava), or to foul (pollute).