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."
To be sad you are in a horrible mental state and usually depressed. Something can cause you to be sad or it’s just hundreds of small things in your miserable life. You wake up knowing that this day will be the same as yesterday and you won’t make any progress socially, meeting new people. To be sad you usually can’t find any motivation or happiness in your life.