I've notice Old Wives Tales fall into 1 of 2 categories the VAST majority of the time.
1) Stuff we tell kids to get them to do stuff we want them to do, or to stop doing things that annoy us.
2) Stuff that USED to be correct but has either been handed down incorrectly or is no longer pertinent in most cases.
Then theres: Pseudoscience & Superstition. I don't lump superstition in with Old Wives Tales, because OWT usually has a good foundation in fact/ science/ or common sense (or irritating things kids do/won't do), while superstition is usually inexplicable. Take the Romani '1 bird in the house; Someone you care for will die. 2 birds in the house, good luck.' That's superstition. True in every single case in my own life, but superstition nonetheless.
Well... the wet hair in the cold isn't an old wives tale per se (falls into category #2)... it's scientifically sound/ proven that doing so for as short as an hour or two up to long periods of time CAN and does lower your temperature enough to allow an explosive growth of bacteria or viruses that your immune system had previously been fighting off just fine. The head thing is particularly to the point because it's where we lose the most heat AND a major vector for infection (bacteria that is hanging out in your sinuses and throat. It's a MAJOR problem for military operations in chill to cold environments, because spending extended periods of time in cold wet weather (constantly out of doors) triggers epidemic levels of pneumonia and strep. It was PARTICULARLY true in times past, because mycobacterium tuberculosis is a 'time bomb'. The bacteria HIDES in the lymphatic system until the immune system is weakened or it's triggered in some other way. A huge percentage of people used to be walking around with dormant TB.
It's not so true in MODERN times, because people go from warm house, to warm car, to warm building. People these days are rarely wet and out of doors for longer than 10 minutes, and mostly for merely the few seconds it takes to go from warm house/ warm car/ warm building. AND we have immunizations... so pneumonia is one of the very few serious illnesses that we contract. But in years past all of the deadly diseases were still very much out there, and people had to be very careful with their health. These days, who cares about a cold or the flu? (Well, the 50,000 people who die of the cold & flu in the US every year). AND we have hair dryers. Women used to normally have waist length hair as the norm. Ever dried waist length hair? It takes a LONG time. The phrase "I'm washing my hair" used to be a valid 'excuse' because it was a task that took several hours due to washing, detangling (before conditioner), and drying. Brushing your hair in front of a fire to get it completely dry was serious business. Esp before central heating.
But 100 years ago (and for military in winter weather) if you went outside in the cold while being wet, you STAYED cold and wet for hours. In one of my schools (this dates me) we were trained for winter ops. One had to be VERY careful about not getting wet if it could be avoided, because getting wet meant you got hypothermia at best and sick at worst. The colder it is the more dangerous getting wet is. To the point that in snow environments, when you shovel snow, you do so NAKED. It's so your sweat doesn't get your clothes wet. You strip down, shovel out the snow from your entrance tunnel, come back in, rub yourself down, and get dressed again. Brrrr. But the heat from shoveling the snow keeps you warm enough to be safe even though it doesn't FEEL warm, it's a lot safer than sitting in clothes damp from sweat in severe cold.
okay, stick in the mud part over
Pregnant women shouldn't bathe (although this can be correct for certain regions historically speaking, again, cat2)
Drinking water dilutes your stomach acid so you can't digest your food
Butter on a burn ((again though, this was HISTORICALLY correct... butter used to be kept in the ice cellar, wrapped in wax paper or in clay jars. People wrapped the jar or frozen butter in a cloth and put it on a burn, which is the right thing to do, cool a burn immediately. AKA it was like how we put frozen peas, or an icepack out of our freezers today, wrap to keep from damaging our skin, and apply. Frozen butter -or steaks- were icepacks prior to ice packs. Ice was FAR too valuable to waste, but butter or steaks could be cooked up once they defrosted. Over time, people misunderstood the practice. Similar to boiled cow's urine. Boiled cows urine becomes VERY antibacterial very quickly, and for centuries has been made into a paste for open cuts. Well, in a certain part of africa in ONE particular region, that info got screwed up. Instead of urine, they use feces. Which ups their death rate tremendously as cow poo is RIFE with e. coli))