To giggle quietly or to chuckle softly; to laugh in a subdued, often secretive way.
授業中に後ろの席の二人がくすくす笑っていた。
Two people in the back row were giggling during class.
彼女の冗談に皆がくすくす笑った。
Everyone giggled at her joke.
子どもたちはくすくす笑いながら秘密の話をしていた。
The children were whispering secrets and giggling.
彼はくすくす笑ってから、冗談の説明を始めた。
He giggled and then began to explain the joke.