This word can be used in place where "RIP" is typically used. Used to express your disappointment or sadness towards what someone is saying to you in one word.
Ex. 1:
"Hey, I have to cancel tonight's plans. I'm really sorry, we can go out another time?"
"Sad."
Ex. 2:
"Are there any cookies left?"
"No, I ate them all."
"Sad."
Ex. 3:
"What did you end up getting on that essay?"
"0"
"That's too bad"
"Yeah. Sad."