Mero-mero — an onomatopoeic word meaning to be madly in love or totally infatuated with someone/something.
彼は彼女の笑顔を見るたびにメロメロになる。
He becomes completely infatuated every time he sees her smile.
子犬を抱いたら、みんなメロメロになってしまった。
When they held the puppy, everyone became totally smitten.
あの俳優の演技にファンはメロメロだ。
Fans are crazy about that actor's performance.
Also used to describe being exhausted or limp — feeling worn out or collapsed (context-dependent).
徹夜が続いて、彼は朝からメロメロだった。
After consecutive all-nighters, he was exhausted from the morning.
運動後は体がメロメロになってしまったが、気持ちは爽快だ。
After exercising, my body felt worn out, but I felt refreshed.