Any leads on a metal mug that might fit in a pocket?

I wish - If preppers ever find such an item, it would literally be the Holy Grail. In the past, there have been various folding, collapsing cups, but none of them have stood the test of time.

The best I can do is a cup that nests around my canteen.

When you say "fit in a pocket," what kind of dimensions or capacity do you mean?

The only flat item I can think of is a heavy foil baking pan. Durability is always the question mark with these.

A different approach might be the classic Sierra cup. These are light, durable, and extremely useful.

The 10-ounce version is 1-1/2" deep. I find it slips neatly into a cargo pants pocket or even the back pocket of my jeans without being overly annoying.

The 16-ounce version is deeper, but IMO would still be okay in a cargo pocket. I sometimes slip one under my belt during impromptu explorations and don't even notice it's there.

my first thought was a metal flask with the top cut off

it really depends on what you are wanting to do with this as one way or the other it seems likely you'll have to compromise. Something that will fit in your pocket (like the flask) means you'll likely have to give up some carrying capacity; on the other hand, something with decent carrying capacity likely won't be pocket-size

The trouble with these are the seams around the bottom. Non-sylindrical shapes are typically not stamped out of a single sheat of metal, but out of strips of metal that are welded or soldered together. This soldering/welding may very well contain nasty stuff that you really don't want to heat up and ingest.

I don't know how much weight to put into this: As long as there is liquid water in it, the inside of a cup won't be heated above 100C. Probably not hot enough for nasty stuff to start polluting your brew in large quantities. Long term exposure effects to harmful chemicals is probably not relevant for rarely used emergency items.

Anyway, since no one really knows what's in those blotched seams I have a dislike against boiling my water in a hip flask.YMMW.

If someone can dig up the old thread "... and a cup" you'll have 13 pages or so of extensive discussion about this and other ingenious solutions....

Survival Resources and others have flat metal "tins" for kit use, etc.

A sierra cup would fit in a large coat pocket. This one is from Evernew and is available on eBay. It's a bit pricy at $26, but you could get a SS one for $5.

Careful With Metal Mugs and such and inspect them in the store if its capable. My Local Gander mountain sells tiny SS sierra cups for like...I believe around 8$? I forget the price off my head but it was super expensive for what it was. The Cups small and the Rolled edge is as sharp and jagged as a tin can that's been opened with a brick. Not something I wanna be putting to my lips.

These are terrific cups. I have several of the titanium Evernews that REI used to sell - I use them for my dog's watering cup on hikes/walks and on the road. In that way we always at least have a metal "pot" with us.

They are very durable and super lightweight.

The stainless versions are still pretty light and very compact and are what I've put in some survival kits that I've built and given away.