Her boyish innocence made her seem younger than she actually was.
enthusiasm
Frequency:
60
%
His boyish enthusiasm for the project was contagious.
grin
Frequency:
50
%
He flashed a boyish grin as he walked into the room.
Idiomatic Expressions for boyish
Phrase: boyish grin
Meaning: a wide, innocent smile that is characteristic of a young boy
Usage: Despite his serious demeanor, he couldn't help but break into a boyish grin when he saw his favorite dessert on the table.