Somebody is trying to brute force the password
I have two ideas to help with this problem.

There are tools and services you can use to look for brute force attacks. The security review module and droptor tools both look in your watchdog (at Administer > Reports > Recent log messages) to see if you have a lot of failed logins for one user. You can also do that manually.

In Drupal 7 the "access rules" feature was removed from core but moved to a contributed module - User Restrictions. Using that module you can block users from a specific IP address. Apache deny rules will be slightly faster if you have relatively small number of rules but the Drupal rules are easier to add/update/remove.