madly in love; head over heels; infatuated
Describes being completely captivated by someone, often in a romantic or doting way. Used with に and な, e.g., メロメロになる, メロメロな様子.
He is head over heels in love with her.
I was completely smitten when I saw that puppy.
collapsed; out of spirits; exhausted
Describes being physically or emotionally drained, limp, or unable to function. Less common than the love sense; often used with に, e.g., メロメロに疲れる.
I was completely exhausted after staying up all night.
Mimetic word; the exact origin is uncertain. The reduplicated form メロメロ likely imitates a state of melting or losing shape, figuratively extended to emotional collapse or infatuation.