To Johnson it was only a verb. He posited Dutch origins, but the etymology is obscure - perhaps from Longinus, the lanky centurion who speared Christ. As a noun, it was a Regency phenomenon (Bath, not the motels of Williams's dread), but the OED does not make clear a nuance nimbly expressed by Martin Fagg in a New Statesman parody of Evelyn Waugh: "I have had to chastise my son for using lounge of a room in a private house." Rest assured, there is no Andy Williams in my sitting-room; Tony Bennett, yes.
THE NEW vogue for the LP In the Lounge with Andy Williams surprises nobody more than the singer: "I used to dread the lounge. If you didn't make it, you ended up singing in the lounge. You know, `lounge' isn't where you want to be."