Meaning:
“肥” means “fat,” “宅” means “home,” “快乐” means “happy,” and “水” refers to “water.” “肥宅” is a recent term coined by Chinese netizens to refer to those who are fat and prefer to stay home watching shows or playing games rather than go outdoors and socialize. This drink, reportedly the favorite of this group, is actually coke, whose high sugar content not just makes people feel happy but can make them grow fat as well.
Example:
A: 你少喝点可乐，含糖量太高。
Nǐ shǎo hē diǎn kělè，hántángliàng tài gāo。
You’d better cut on your coke intake. It has very high sugar content.
B: 既然喝水都胖，那我干嘛不喝肥宅快乐水?
Jìrán hēshuǐ dōu pàng，nà wǒ gànmá bù hē féizhái kuàilè shuǐ?
I will still be fat even if I drink water, so why do I give up this fattie’s happy drink?