IPA: /ˈoʊvərˌlɜrn/
KK: /ˈoʊvərˌlɜrn/
To study or practice something more than necessary after you have already learned it well, in order to make sure you remember it better.
She decided to overlearn the material for the exam to ensure she would not forget it.
Past: overlearned
Past Participle: overlearned
Overlearn is formed from "over-" (meaning excessively) and "learn" (from Old English "leornian", meaning to acquire knowledge). The word means to learn something excessively or beyond the necessary level.
Think of learning something 'excessively' ('over-') — that's why overlearn means to learn too much.