Markedly; noticeably — used as an adverb to indicate a clear or sudden change that is easily observed.
冬になってめっきり寒くなった。
Now that it's winter, it's become noticeably colder.
最近、仕事が忙しくてめっきり休みが減った。
Lately work has been busy and my days off have noticeably decreased.
子供たちはこの半年でめっきり背が伸びた。
The children have grown noticeably taller in the past six months.