"心塞 (xīnsāi)" has become a popular buzzword these days. Here "心 (xīn)" is "heart" and "塞 (sāi)" literally means "blocked" or "plugged", so put together, "心塞 (xīnsāi)" is used to vividly describe a kind of emotion which makes you feel suffocated and uncomfortable. If something gets worse and you are left feeling helpless or speechless, you can say that you feel "心塞 (xīnsāi)".
For example, your mood could be described as "心塞 (xīnsāi)" when the weekend finally comes and you have big, exciting plans, but it ends up pouring down rain and you've no choice but to cancel. Or you are all dressed up for your date but a car drives through a puddle and splashes mud all over you. Your feeling at that moment is "心塞 (xīnsāi)." Or your boss remarks that your proposal, for which you have stayed up working on for several nights, is of no use, so your frustration can also be described as "心塞 (xīnsāi)".
The complete version of this hot Chinese phrase is "心肌梗塞 (xīn jī gěng sè)", but is commonly used as "心塞" for short. "心肌梗塞" literally means heart attack, and has become popular this year amongst netizens. It is less about physical pain but is often used to describe an extremely painful or negative feeling caused by something else.
“心塞”这一流行语是“心肌梗塞 (xīn jī gěng sè)”的简称。“心肌梗塞”本意是指心脏病发作，后来为网友广泛使用，用于指因发生了某事心情变得极度痛苦和消极，并非指真正的肉体上的疼痛。
The character "塞" has two meanings and two pronunciations. The first meaning that can be seen in "心肌梗塞"(heart attack) is "to stop up" and uses the pinyin "sè". However, often when netizens play on words to create new meanings the pronunciation may be changed to better suit the feeling or intent. So, once the original "心肌梗塞" is abbreviated to "心塞", the pronunciation of "塞" becomes "sāi". This is done usually to suggest the phrase or slang is less serious than its actual meaning (to have an actual heart attack) and also becomes more comfortable to speak.
“塞”有两种意思，两种读音。第一种是同“心肌梗塞”里的“塞”的意思一样，意为“堵住（to stop up）”，读音为“sè”。然而，网友在玩文字游戏时，通常会采用新的意思和读音，以更好地表达情感。因此，当“心肌梗塞”简称为“心塞”的时候，塞的读音由“sè”变成“sāi”，表明和真实意思（心脏病发作）相比，这个意思并不严重。这样改过之后，说起来也更舒服。
A：nǐ jīntiān zěnme chídào le?
Why are you late today?
B：Lùshang dǔchē, hǎo xīnsāi!
I got stuck in a traffic jam. How terrible!
A：nǐ zài zhǎo shénme?
What are you looking for?
B：Xīnsāi, wǒde shēnfènzhèng bújiàn le.
Oh crap, I can't find my ID card.