adverb
Emphasizes that something has happened again, often with a nuance of surprise or exasperation. Similar to また, but stronger and more emotive.
またもや遅刻してしまった。
I ended up being late yet again.
彼はまたもや同じ間違いを繰り返した。
He repeated the same mistake once again.
Derived from また (again) with the emphatic particles も and や. The exact historical development is uncertain, but it likely arose as a colloquial intensifier.