偷偷 (tōu tōu) means 'secretly, stealthily' and emphasizes doing something in a hidden or sneaky way, often with a sense of wrongdoing or furtiveness. It differs from 悄悄 (qiāo qiāo 'quietly'), which focuses on silence rather than secrecy, and from the adverb 偷 (tōu), which is more informal and can also mean 'steal' as a verb.