Definition of silly noun from the Oxford Advanced Learner's Dictionary

 

silly

 noun
noun
BrE BrE//ˈsɪli//
 
; NAmE NAmE//ˈsɪli//
 
 
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(British English also silly billy) [singular] (informal) often used when speaking to children to say that they are not behaving in a sensible way No, silly, those aren't your shoes! Word Origin late Middle English (in the sense ‘deserving of pity or sympathy’): alteration of dialect seely ‘happy’, later ‘innocent, feeble’, from a West Germanic base meaning ‘luck, happiness’. The sense ‘foolish’ developed via the stages ‘feeble’ and ‘unsophisticated, ignorant’.
See the Oxford Advanced American Dictionary entry: silly