(as) timid as a mouse
Very nervous, shy, or fearful. I can't believe John actually had the temerity to demand a raise. He's always been timid as a mouse! I love singing, but it's hard to pursue it professionally when you become as timid as a mouse up on stage. Because Molly's as timid as a mouse, she froze when she realized she'd have to speak in front of the entire class.
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