rofi stores a history of previously used commands.

This history is used to determine the order in which commands are displayed.

rofi command history is stored in plaintext under the user's cache directory as rofi-3.runcache

On most systems the full path will be ~/.cache/rofi-3.runcache

Note that these files are generated and should not be edited. The format can change at any time.

rm ~/.cache/rofi-3.runcache

See man page