In a way that is very noticeable, impressive, or surprising; standing out clearly.
From 'strike' (to hit or be noticed) plus '-ing' forming a participle, then '-ly' making it an adverb. The metaphor of something 'striking' your attention like a blow has been used since Middle English.
The word 'strike' originally meant a physical blow, but by the 1600s people used it metaphorically for anything that 'hits' your consciousness hard—a strikingly beautiful sunset doesn't literally punch you, but it does grab your attention powerfully.
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