In many scenarios, you may be using a symbol very often. For example, new Indian Currency Symbol ₹. Every time, you need to insert symbol which is many steps process and introduces inefficiency. You can use following method to assign a short-cut key to a symbol. (Below example is actually not a short cut key but a word which you type)
Let's take example of ₹.
1. Insert this symbol in an empty cell and copy this symbol. You need to copy only the symbol by selecting and CTRL+C. Don't copy the entire cell.
2. Go to Excel Options > Proofing and Click Autocorrect Options (For Excel Options, you need to press Office Button and Excel Options is right bottom.
3. Put a word for the symbol in Replace box and paste (CTRL+V) and paste symbol in With box. (It is suggested that you enter lower case letters)
4. Now whenever you enter the word IRC in a cell, the symbol will be inserted.