Explanation
Also expresses an extreme degree or state, typically negative.
Formation
な-Adjective（語幹）+ 極まる, い-Adjective（連用形）+ こと + 極まる, etc.
Detailed explanation
The ～極まる form often appears with な-adjectives: e.g., 失礼極まる, 非常識極まる ('extremely rude/unreasonable'). It's largely synonymous with ～極まりない but tends to appear in set phrases. Both forms are quite formal and emphatic.
Examples
彼の言動は失礼極まる。
Kare no gendou wa shitsurei kiwamaru.
His words and actions are extremely rude.
その態度は非常識極まる。
Sono taido wa hijoushiki kiwamaru.
That behavior is utterly irrational.
彼女の発言は無責任極まる。
Kanojo no hatsugen wa musekinin kiwamaru.
Her remark is extremely irresponsible.
このような誤解は遺憾極まる。
Kono youna gokai wa ikan kiwamaru.
Such a misunderstanding is exceedingly regrettable.
Practice
Grammar content sourced from hanabira.org.