You Should Never Put Frozen Meat In Your Slow Cooker. Here's Why
Since the ingredients in a slow cooker simmer and stew away forever, it would stand to reason that you should be able to put anything in there, even a chunk of completely frozen meat.
However, if you've ever taken a food safety class in order to obtain a food handler's permit, you know that one of the most important concepts when it comes to handling foods is keeping them out of the "danger zone."
The danger zone is the range of temperatures at which dangerous bacteria such, as E. coli, salmonella, and campylobacter can grow on foods, especially raw meat, poultry and seafood. The range of 40 degrees Fahrenheit and 140 degrees Fahrenheit are defined by the United States Department of Agriculture as the danger zone. (via USDA). While it's inevitable that food has to spend some time in the danger zone to go from the refrigerator to fully cooked, the trick is to minimize the amount of time spent in that range in order to avoid making a dangerous cooking mistake.
What happens to frozen meat in a slow cooker
Unfortunately for slow cooker fans, these nifty devices cannot be relied upon to move the meat through the danger zone in a speedy manner (which shouldn't be too surprising if you think about the name (via Two Healthy Kitchens). The gradual heat with which a slow cooker works results in the dish spending too much time between those dangerous temperatures. There are, of course, plenty of people who have cooked frozen meats in the slow cooker and lived to tell the tale, but it seems like it shouldn't be worth the risk.
Your best bet is to either defrost the meat that you'll be using in the microwave, under cold running water or in the refrigerator overnight, which is generally agreed upon to be the best and safest option, although it does require some degree of planning ahead—and a lack of foresight is likely what got you into the frozen meat slow cooker predicament in the first place (via Good Housekeeping).