嘿嘿 is an onomatopoeia for a sly, mischievous, or self-satisfied laugh — like a quiet 'hehe' or 'tee-hee' in English. It often conveys teasing, secret amusement, or mild gloating, and is common in informal speech and text messages. Unlike 哈哈 (loud, hearty laughter) or 呵呵 (polite or awkward), 嘿嘿 has a playful, knowing tone.