Once-upon-a-time when I had a microwave, I mostly used it for defrosting meat. It is a quick, but not very satisfactory, method. The outside can cook a bit while the inside thaws (especially noticeable with chicken as I recall). Microwaves don’t penetrate more than about 1cm into the food, so defrosting relies on heat from the edges of the meat conducting into the core to complete the job (for thicker pieces). It’s a fallacy that microwaves cook food from the inside out.
It’s a relatively food-safe method, if done with care. But there are potential hazards. Mainly that the outside of the food runs the risk of being heated to a bacteria-friendly temperature (~30-40C), and held there, while heat is conducted into the interior to complete the defrosting.
A very safe method is to put the frozen meat in the fridge. It will defrost (eventually), but never exceed ~3C (the temperature of an average fridge), so it remains safe. The problem is that it can take a day or more to achieve this.
It takes that long to defrost in the fridge because air is very poor at transferring heat. I have briefly posted on this before, but to recap:
Imagine your oven has been heated to 100C, and that you have a pot of boiling (100C) water on the stove. The air in the oven and the water on the stove are both at the same temperature (100C) – but which one would you be prepared to put your hand in?
So, it can take ages for the air in the fridge to transfer heat to the meat and defrost it.
The worst thing to do is to try and speed this up by putting the meat on a counter-top at room temperature. And it still takes ages anyway.
The best and safest method is to put the frozen meat in an ice-water bath. This works for the same reason that you didn’t chose to put your hand in the boiling water (I hope) – water is a great conductor of heat and will thaw the meat quickly.
Method: Run some cold water into a bowl or small sink, and add plenty of ice (a 50:50 ratio is ideal). If the meat was vacuum packed before being frozen (recommended), then just pop the bag in the bath. If not, take the meat out of whatever it was wrapped in, and put inside a waterproof plastic bag (something like a ziplock) and leave the top open. Slide the bag into the water keeping the opening above water level, and use water pressure to expel most of the air from the bag around the meat. You want as little air left as possible so that most of the meat is in contact with the water (via the plastic) and the meat does not float. Let sit until thawed – I predict you will be surprised how fast and effective this is.
The water in the ice-bath cannot be below 0C (or it would be ice), but is kept cool by the ice (it usually sits around 3C – fridge temperature). So it is equivalent to thawing in the fridge, but using water instead of air and so very much faster.
You might think that warmer water would be better, but surprisingly it isn’t that much faster, and of course the outside of the meat is no longer at a safe food temperature.
By the way, an ice-bath is a great way to cool cooked meat before refrigerating it. As much care should be taken with cooling as with defrosting. If the meat is allowed to cool on the counter slowly, it spends more time in the bacterial danger-zone. Putting the hot meat in the fridge is also not a good idea as it warms part of the fridge and affects surrounding food items. It is also still slow to cool.