Definition of lightning adjective from the Oxford Advanced Learner's Dictionary

 

lightning

 adjective
adjective
BrE BrE//ˈlaɪtnɪŋ//
 
; NAmE NAmE//ˈlaɪtnɪŋ//
 
 
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[only before noun] very fast or sudden to move at lightning speed Word Origin Middle English: special use of lightening (verbal noun from lighten in the sense ‘to become lighter’ or ‘brighter’).
See the Oxford Advanced American Dictionary entry: lightning