IDK stands for I don't know. People use it in texting and on social media when replying quickly and informally, without spelling out the full phrase.
Could you drop idk into a text?
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idk + what/why/if…; idk, maybe…
- idk what time the meeting starts, check the calendar.
- idk if our train is still on time, I'll check the app.
- idk why he hasn't replied yet, maybe he's just busy.
What changed
First versionDear Professor Smith, idk when the assignment is due, could you remind me?
RevisionDear Professor Smith, I'm not sure when the assignment is due, could you remind me?
IDK is a texting abbreviation, and a message to a professor is a formal piece of writing. Here it's replaced with the neutral phrase I'm not sure, which sounds softer and more polite in a formal message than a blunt I don't know.
On its own, idk can sound a bit blunt, so people often add a guess after it: idk, maybe Thursday? Don't use it in a message to a teacher or colleague, or in an essay; in IELTS Speaking, say I'm not sure or I have no idea instead.
Your turn
Mia asked if I'm free on Friday. I'm not sure yet, so I texted back: ___, I'll let you know tomorrow.
Show the answer
idk — The writer doesn't know yet whether they're free on Friday and promises to answer tomorrow — that's uncertainty, which is exactly what idk expresses. Nah is a casual no, and a flat refusal doesn't fit with promising to reply later.
Write a text to a friend saying you're not sure yet about your evening plans. Say exactly what you're unsure about and when you'll give a definite answer.
idk if I can come to the party, I might have to work late, I'll text you by six.
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