Shouldn't be a problem as long as it's empty. When I had a 3hr journey to the airport in the past my mother used to make me a small flask of coffee which I would drink before getting to the airport. The security staff always had a look at it but I never had a problem.

If you are taking checked baggage then it might be better to put it in your suitcase.

I take one of the insulated cups on the plane for 'sir' and whilst it does get checked at security we never have a problem. I take it out of my bag and put it in the tray with my liquids bag so they can see what it is.