/ ˈsaɪdlɒŋ; NAmE -lɔːŋ/ adjective
[ only before noun ] ( of a look ) out of the corner of your eye, especially in a way that is secret or disapproving :
She cast a sidelong glance at Eric to see if he had noticed her blunder.
► side·long adverb :
She looked sidelong at him.
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WORD ORIGIN
late Middle English : alteration of earlier sideling , from side + the adverbial suffix -ling .