Hi, the WR dictionaries say that one of the meanings of "tumber" is:
a stemless drinking glass having a flat, often thick bottom. (Random House)
a flat-bottomed drinking glass with no handle or stem. (Collins)
So I've just done a quick Google image search to see if they look like ordinary glasses.
Strangely, the vast majority of the containers displayed there aren't glasses. What's going on!?
tumbler - Google Search
If I went to a department store in your country and asked "Excuse me, I'm looking for tumblers.", would they think I'm looking for large glasses, or non-glass, tall mugs with a lid like the ones shown above?
a stemless drinking glass having a flat, often thick bottom. (Random House)
a flat-bottomed drinking glass with no handle or stem. (Collins)
So I've just done a quick Google image search to see if they look like ordinary glasses.
Strangely, the vast majority of the containers displayed there aren't glasses. What's going on!?

tumbler - Google Search
If I went to a department store in your country and asked "Excuse me, I'm looking for tumblers.", would they think I'm looking for large glasses, or non-glass, tall mugs with a lid like the ones shown above?