“They're probably disappointed in me.”

You're having a deep talk with a close friend about your parents. You told your friend that your parents set high standards for you, but you didn't meet them. You say this because you think your parents are probably sad that you haven't been more successful in life.

They're probably disappointed in me.

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(someone is) probably (adjective)

Use "probably" to talk about something that you think is true, but you're not sure. "Probably" describes a probability anywhere from 50% to 99%. Use "probably" with an adjective:

You're probably exhausted.

You can also use "probably" with a longer phrase that acts like an adjective:

He's probably on his way.

And it also can be used with a verb:

They probably left to go get some food.

(someone is) disappointed in (someone)

When you're "disappointed", it means that you're a little sad because something wasn't as good as you expected it to be. You can be "disappointed with" a movie, a sports team's performance, and so on:

Betty was kind of disappointed with how her cupcakes turned out.

But when you say that you're "disappointed in" a person, it means that someone who you used to have a high opinion of did something bad. So now you can't respect that person as much.

The phrase "disappointed in ___" is usually used when you're talking about your children, your employees, or someone else with lower status. If you directly tell someone that you're disappointed, it's a pretty serious statement and makes it sound like you have authority over that person:

Son, I'm disappointed in you.

You might tell someone this if you found out that they did something immoral like lying, cheating on their partner, or stealing something.