Sentence:  At once my ears were drowned by a flow of what I took to be Spanish , but - the driver 's white teeth flashing at me , the road wildly veering beyond his glistening hair , beyond his gesticulating bottle - it could have been the <WSD>purest</WSD> Oxford English I was half hearing ; I would n't have known the difference . 
Meaning of the word marked with <WSD> tags:  free of extraneous elements of any kind