乖乖 is a reduplicated form of 乖 (well-behaved). It has three main uses: as an adverb meaning 'obediently, without resistance'; as a term of endearment for a child, equivalent to 'sweetie, darling'; and as an exclamation (often repeated: 乖乖！) expressing surprise or admiration in casual speech. The tone pattern is neutral: both syllables are first tone (guāi guāi), but in exclamations the second syllable may be lengthened with a strong fall.