When you combine them, the meaning is literal and clear: 不 (not) + 知道 (to know) = 不知道 (to not know).
While “I don't know” in English is a neutral and very common response, its Chinese equivalent, 不知道 (bù zhīdào), carries more nuanced cultural weight, primarily related to the concept of 面子 (miànzi) - face. In Chinese culture, maintaining social harmony and preserving “face” (one's reputation, dignity, and prestige) is very important. A blunt 不知道 can sometimes cause someone to “lose face” in certain situations:
For this reason, native speakers will often “soften” the phrase to be more polite and maintain relational harmony. Instead of a stark 不知道, they might say:
This is a key difference from typical Western communication, which often values directness over indirectness. For a learner, simply being aware of this tendency to soften responses is a huge step toward cultural fluency.
不知道 (bù zhīdào) is used constantly in daily life, but its connotation changes with context and tone.