Meaning:
Literally meaning “big melon,” this term frequently used by Chinese netizens refers to “an explosive rumor or gossip.” This term is an example of back formation, as it is derived from the saying “吃瓜群众” (literally, melon-eating people, which actually refers to those interested in discussions of gossips and rumors). “Big melon” often refers to celebrity gossips.
Example:
A: 据说香港某知名已婚男歌手和一个女演员在出租车里亲热，被拍了视频。
Jùshuō xiānggǎng mǒu zhīmíng yǐhūn nán gēshǒu hé yīgè nǚ yǎnyuán zài chūzūchē lǐ qīnrè，bèi pāi le shìpín。
A well-known married Hong Kong singer was reportedly caught on video making out with an actress in a taxi.
B: 又是一个大瓜，既然有视频，那就是实锤了。
Yòu shì yīgè dàguā，jìrán yǒu shìpín，nàjiùshì shíchuí le。
That’s a big rumor! Now that there is video, it must be true.