Sentence:  He looked at her out of himself , she thought , as he did only for an instant at a time , the look which always surprised her even now when his uncombable hair was yellowing a little and his breath came hard through his nicotine choked lungs , the look of the gaunt youth she had suddenly found herself staring at in the Tate Gallery on a Thursday <WSD>once</WSD> . 
Meaning of the word marked with <WSD> tags:  on one occasion