Meaning:

    “脑洞” means “a hole in the brains,” and “大开” means to “open wide.” A derivative from another term “脑补” (literal meaning: fill in one’s brains) often used in Japanese anime meaning to “imagine and fantasize,” this one refers to basically the same thing, but with upgraded intensity. The logic is: The bigger hole you have in your brains, the more stuff you can fill in. Therefore, this term means to “fantasize wildly.” Chinese netizens often use it in contexts where people come up with absurd and hilarious responses to stuff.

    Example:

    有个富二代在网上买了一张两百块的电脑桌抱怨送货慢，令网友脑洞大开，想象出各种剧情。

    Yǒu gè fù’èrdài zài wǎngshàng mǎi le yīzhāng liǎngbǎikuài de diànnǎozhuō bàoyuàn sònghuò màn, lìng wǎngyǒu nǎodòngdàkāi, xiǎngxiàng chū gèzhǒng jùqíng。

    A guy who was born loaded buys a 200-yuan computer desk online and complains about late delivery. This gets netizens to use their imagination and come up with various explanations.

    可能你付成两百万了，他们正在给桌子镶金。

    Kěnéng nǐ fù chéng liǎngbǎiwàn le, tāmen zhèngzài gěi zhuōzi xiāngjīn。

    Perhaps you paid 2 million by mistake, and they are making you a gilded version.

    我就是送快递的，你的房子太大我迷路三天了。

    Wǒ jìushì sòng kuàidì de, nǐ de fángzi tài dà, wǒ mílù sāntiān le。

    I’m the delivery guy. Your estate is too big and I’ve been lost in it for three days.