不X白不X: might as well, it's a waste not to
【四67】口语格式 / fixed patterns
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
不X白不X is what you say when a chance is free or a bargain: if you don't take it, it's a waste. The same verb is repeated on both sides of 白: 不吃白不吃, 不拿白不拿, 不看白不看. English: “might as well”, “it'd be silly not to…”.
Here 白 (bái) means “for nothing, in vain”: not doing it means letting the chance go for nothing. So the pattern needs a real opportunity (a free meal, a sample, a discount); with something expensive or unpleasant it sounds wrong.
Register: spoken and playful, between friends. Compare 既然……就……: 既然是免费的，就吃吧 draws a calm conclusion; 不吃白不吃 adds a wink, “it would be a pity”.
Examples
免费的，不吃白不吃！
Miǎnfèi de, bù chī bái bù chī!It's free, we might as well eat!
老板送了我们一碗汤，不喝白不喝。
Lǎobǎn sòng le wǒmen yì wǎn tāng, bù hē bái bù hē.The owner gave us a bowl of soup, so we might as well drink it.
这部电影免费看，不看白不看。
Zhè bù diànyǐng miǎnfèi kàn, bú kàn bái bú kàn.The film is free to watch, so it would be silly not to.
公司请客，大家都去，不去白不去。
Gōngsī qǐngkè, dàjiā dōu qù, bú qù bái bú qù.The company is treating everyone and everybody is going, so why not go?
今天商店打五折，不买白不买。
Jīntiān shāngdiàn dǎ wǔ zhé, bù mǎi bái bù mǎi.The shop is half price today, so it would be silly not to buy.
试试又不花钱，不试白不试。
Shìshi yòu bù huā qián, bú shì bái bú shì.Trying costs nothing, so we might as well try.
Common mistakes
✗ 不吃不白吃 → ✓ 不吃白不吃 (白 sits between the two “不 + verb” halves).
✗ 这个电影很贵，不看白不看 → ✓ 这个电影是免费的，不看白不看 (the pattern needs a free or cheap chance).
✗ 明天考试，不复习白不复习 → the pattern is for grabbing a bargain, not for talking about a serious duty.