Onomatopoeic adverb meaning to speak in a whisper or under one's breath — talking quietly so others can't hear.
図書館でヒソヒソ話すのはやめてください。
Please stop whispering in the library.
子どもたちはヒソヒソと秘密の話をしていた。
The children were whispering a secret.
二人は廊下の隅でヒソヒソと相談を始めた。
The two began to whisper and consult in the corner of the hallway.