This is the 1018th most frequent Japanese word.
まい
The Japanese word "まい" can mean "every" (e.g., 毎朝 - every morning) or is a counter for flat objects (e.g., 一枚 - one sheet).
In this sentence, 'まい' is used as a negative volitional form, indicating an intention or decision not to do something.
彼はそれをするまい。
He will not do that.
Here, '毎' represents 'every,' indicating frequency, as in '毎日' meaning 'every day'.
毎日走っています。
I run every day.
In this context, 'まい' forms part of 'まいしてくれた,' a classical verb form indicating graceful or willing performance.