Meaning:
“笑” means to “laugh,” “出” means to “appear,” and “腹肌” means “six packs.” This term coined by Chinese netizens is an exaggeration where the speaker literally says one laughs so card that he feels his six packs get some good exercise. It’s often used when people want to say that something is hilarious.
Example:
A: 我前几天关注了一个微博帐号，博主原创的一个说未来中国足球队世界杯夺冠的帖子，简直让我笑出腹肌了。
Wǒ qiánjǐtiān guānzhù le yīgè wēibó zhànghào，bózhǔ yuánchuàng de yīgè shuō wèilái zhōngguó zúqíuduì shìjièbēi duóguàn de tiězi，jiǎnzhí ràng wǒ xiàochū fùjī le。
I followed a Weibo account the other day, and the owner created a post describing the Chinese soccer team winning a World Cup in the future. It made me laugh very hard.
B: 你说的那个帖子我也看了，的确很好笑。
Nǐ shuō de nàge tiězi wǒ yě kàn le，díquè hěn hǎoxiào。
I read that post too. It’s hilarious indeed.