What is "hand hot"?

I don't know the term either. I don't think it is established baking slang, so it is bound to vary between recipes, should you find it in another one at all. But if you got it from a bread recipe, it must be because you need optimal conditions for your yeast. The optimal temperature for yeast rising is 35°C, with rising being too slow below that (but it will still happen, even at 4°C in the fridge!) and not possible at 40°C and above, where leavening action gets too low for practical purposes (and at some point, the yeast dies).

This is a nice representation of the amount of CO2 produced by yeast (which correlates well with leavening) at different temperatures. The difference between the low effective temperature (25°C), the optimal temperature (35°C) and the upper limit of the effective temperature (40°C) is not big, so I don't rely on my imperfect senses and always use a thermometer when making yeast dough.

But you are writing that you want a "rule of thumb", so you probably don't have (or don't want to bother with) a thermometer in your kitchen. In this case, you can still have your bread rise well. The literal thumb is a bad choice, as it is quite insensitive, but the trick our grandmas used to gauge the temperature of infant food is still valid: use your elbow.

The skin of the elbow is very thin, and it will very well notice the difference between a 35°C liquid (which feels roughly the same temperature as the elbow, remember that 37°C are normal inside the body, not on the skin outside) and a 40°C liquid (which feels too warm). If you were to use your fingers or the back of your hand, you wouldn't be able to tell the difference well enough, and will risk overshooting and killing the yeast. There are possibly other body parts which can feel the difference (I'd guess the tongue, if you don't scald it daily with hot drinks), but hygienewise, the elbow is probably better.

I've never seen that term before, but from the context of a bread recipe, it must mean "body temperature", i.e. around 100 F / 37 C. Water that you could just barely hold your hand in (around 140 F) would kill the yeast instantly.

I suspect this might be a UK-ism, it usually crops up in bread and cake recipes and is fairly well known on these shores. You're right to guess that it's to do with yeast-based baking - as has been scientifically pointed out above, yeast likes warm but not hot water.

Hand-hot is an easy shorthand for "warm enough that you can put your hand in it with no discomfort" and is a little less grisly than the also-used "blood temperature".

"Hand hot" is the hottest temperature you can immerse your hand in without pain. Generally, this is about 110-115F, or 43-46C. If you have calloused, tough hands, measure this by running water over the thinner skin on the back of your hand. The idea is to get the dough warm so the yeast works faster, but not so hot that it rises too fast or kills the yeast. Once you combine the water with room temperature flour, the result is close to the optimum rising temperature for yeast.

Anyway, temperature doesn't need to be super-precise when you're judging the rise by the feel and volume of the dough. "Hand hot" is what we use to prepare dough at the restaurant I work for, and it gives good results. Plus it's super-fast, which is important.

My advice as a chef is to get a thermometer.

If you are working in a kitchen that is hot (>30° c) then have the water yeast and sugar "activater" mix at 35deg c. That is about the temperature that feels hand warm in a hot kitchen.

In a cool kitchen (<30°c) then I have the "activater" mix somwhere between 40°c -43°c. This allows the dough to maintain a close to optimum temperature once energy is lost to the cooler bowl and flour etc.

Thats assuming I don't have access to a good proving oven. In which case start up temps are quickly over come by the warm and humid environment in the proving oven.

If course with the above your individual experience may vary due to many other factors as well. So ultimately to make great bread takes lots of practice.

Thats why bakers spend years in an apprenticeship.