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remarkably; noticeably; considerably; appreciably
秋になってめっきり涼しくなった。
It has become considerably cooler now that it's autumn.
忙しくなってから最近めっきり友人と会う機会が減った。
Lately opportunities to meet have remarkably decreased.
祖父はめっきり老け込んでしまった。
My grandfather has aged remarkably.