“You wouldn’t know”: If you ask to roll, say, history, and your DM responds with this, it usually means, “This is a vital plot point that you aren’t supposed to find out until later, so I won’t tell you regardless of what you roll”
“I mean, you could”: The DM is strongly suggesting you don’t do the thing you were just about to do. But it is your choice… if you reeeeeally want to.
“Oh, fuck”/“Oh, shit”/et cetera: “I DID NOT EXPECT YOU TO MAKE THAT DECISION AND I DID NOT PREPARE AN OUTCOME”
“Oh, fuck”/“Oh, shit”/et cetera: “I JUST REALIZED I DID NOT BALANCE THIS COMBAT CORRECTLY”
“Hang on…”: “Where the FUCK did I put this in my notes?”
“Oh boy”/“Oh god”/“Oh no”/et cetera: I either just rolled REALLY well or REALLY badly. You’ll find out soon enough.
“It seems like…”/“As far as you can tell…”: What I’m about to say your character notices, is nowhere CLOSE to what’s actually going on.
“Make a [skill] check”/“Make a [skill] saving throw”: I’m having you make this roll, but I’m not going to tell you what it means until later, when you’re going to regret it.
A hard, firm, “No”: “Please for the love of GOD and ALL that is holy I am BEGGING you not to put me through whatever BULLSHIT you’re planning.”