By now I’m sure that you’ve used or last seen Excel’s formula autocomplete.
While you could go to the File tab / Options choice to turn it on or off, it’s much faster to do it right in the worksheet.
All you have to do is use Alt + Down Arrow while in a cell of the worksheet.
This is a toggle combination so Alt + Down Arrow again will turn it back on.
On – Off – On – Off… it’s as simple as a key combination without a single trip to the Options dialog.