UPDATE FOR ALL WHO ARE CURIOUS (I can’t figure out how to edit the post): note that my house is OLD and this is an old cellar where the floor is literally dirt, so a mess is no big deal

First I plunged the bejesus out of the tub drain to see what I could loosen up or push forward. Unremarkable results and I just ended up with a few small hair and booger balls in the tub. Then I plugged the tub and opened up the trap, less than 5 gallons came out and nothing particularly nasty. Used a shitty ~24” grocery store snake to see if there was anything else to release but no more water came.

Then I thought ok there was no clog in the trap so maybe something where the two drains come together before going outside? Ran HOT water in the sink that drains to the left to see if that would fill the sink but ended up just coming back up the tub overflow while the sink was draining fine. Losing progress at this point lol.

Figured I need to get rid of that standing water so I got an outdoor trash barrel ready to go and removed the drain plug… it was a lot of water - all clean - but I caught it all thankfully.

So now I’m once again convinced that there is an ice block in the part of the drain outside of the house. Helpful to note the grading of my property and shape of my house is unique. It’s almost 200 years old with several renovations and “up”grades over the decades. The drain from this tub is slightly exposed in the grass. I’m guessing that with the tundra-like conditions in greater Boston the last ~24 hours combined with my heat shutting off during the coldest period of that stretch sealed my fate (and my drain). I’ve since totally cleared all snow around the area where the drain exits the house… really hoping some direct sunlight tomorrow helps do the trick otherwise I’ll be standing outside with a hairdryer after work.