The Extreme Emphasis Pattern: ~極まる / ~極まりない (Kiwamaru / Kiwamarinai)

• ~極まる ({きわまる}|kiwamaru): This is a verb meaning 'to reach an extreme'. It feels slightly more literary or archaic to some speakers. For example, 彼の態度は、横柄極まる ({かれのたいどは、おうへいきわまる}|kare no taido wa, ōhei kiwamaru) - 'His attitude is the height of arrogance.' It presents the state as a finalized peak.
• ~極まりない ({きわまりない}|kiwamarinai): This form uses the ~ない suffix, which in this context means 'without limit'. It's a form of emphatic negation, suggesting the quality is so extreme it's immeasurable. その要求は、不当極まりない ({そのようきゅうは、ふとうきわまりない}|sono yōkyū wa, futō kiwamarinai) - 'That demand is unjust to a limitless degree.' This form is more common in contemporary formal Japanese, especially in business or political discourse, as it carries a potent, assertive force.

For the most part, yes. They carry the same meaning of 'extremely'. However, ~極まりない is more common in modern formal Japanese (especially in speech, like news or political commentary) and can feel slightly more emphatic due to the 'limitless' nuance. ~極まる can sometimes sound a bit more literary or stiff. When in doubt, ~極まりない is often the safer and more contemporary choice for negative descriptions.

Yes, that is the safest way to think about it. The overwhelming majority of use cases involve describing negative qualities, states, or actions (rude, dangerous, foolish, unpleasant). The primary positive exception you must know is the set phrase 感極まる (kan kiwamaru - to be overcome with emotion). While you might encounter other positive uses in literature (e.g., 平穏極まる - heion kiwamaru - extremely peaceful), they are rare. As a learner, you should stick to using it for negative emphasis.

~の極み (no kiwami - 'the peak of...') is a very close cousin. The main difference is grammatical. ~の極み is a noun phrase that attaches to other nouns, often to describe the height of a luxurious or abstract concept (e.g., 贅沢の極み - zeitaku no kiwami - 'the height of luxury'). ~極まる attaches to adjective stems. While ~極まる is mostly negative, ~の極み can be used for both positive and negative extremes.

The pattern itself is not rude; it is a feature of formal language. However, it is used to express strong, often negative, judgment. Therefore, directing a phrase like 失礼極まりない ('extremely rude') at someone is a very direct and powerful accusation. It is not inherently impolite, but it is inherently confrontational and should be used only when that level of force is intended and appropriate for the formal setting.