“I know that you've all really busted your asses on this project, and I really appreciate it.”
You're a manager at your company. Your team just finished a big project. You take them out to lunch to thank them. When the drinks arrive, you make a toast to their hard work.
I know that you've all really busted your asses on this project, and I really appreciate it.
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(someone) appreciates (something)
When you "appreciate" something, it means that you notice it and are thankful for it.
You say "I appreciate ___" or "We appreciate ___" when you want to thank someone for their actions. This phrase sounds more clear and explicit than just saying "Thank you for ___".
An example of this is when your child confesses that she broke something, and you say:
I appreciate your honesty.
bust (one's) ass
"Busting your ass" is an idiom that means "working really hard".
I busted my ass waiting tables and working construction to pay my way through college.
I know that all of you have really busted your asses on this project, and I really appreciate it.
This is a colloquial expression that you use when talking to people you're comfortable with. The word "ass" is a mild swear word. You probably shouldn't use it in formal situations.
There's also another meaning of "busting your ass", by the way. It means to injure your butt by falling:
You remember that time when you slipped on the ice in the front driveway and busted your ass?
I know that you've (done something)
You can say "I know that..." to show someone that you understand their feelings:
I know that you were disappointed that we didn't get a chance to visit the winery. I'm sorry about that.
I know that you must have spent a lot of time putting all of these notes together. Thank you so, so much for doing that!