what does "arch look"mean?
Strike had to admit that the coat he had thought so vile looked better on Robin than on the mannequin. She twirled on the spot for him, and the thing glittered like a lizard’s skin.
“It’s all right,” he said, masculinely cautious, and the assistants smiled indulgently. “Yeah, it’s quite nice. How much is it?”
“Not that much, by your standards,” said Robin, with an arch look at her handmaidens. “Sandra would love this, though,” she said firmly to Strike, who, caught off guard, grinned. “And it is her fortieth.”
“She could wear it with anything,” the cotton candy girl assured Strike eagerly. “So versatile.”
“OK, I’ll try that Cavalli dress,” said Robin blithely, turning back to the changing room.