Resembling a flash; occurring very quickly or with sudden bright light.
Compound of 'flash' + 'like' (suffix meaning 'resembling or characteristic of'), a productive word-formation pattern in English.
The '-like' suffix is one of English's most flexible tools—you can attach it to almost any noun to create an adjective, which is why 'flashlike' is instantly understandable even if it's not commonly used.
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