уму непостижимо

the mind boggles (Putney Heath); it defies comprehension (Taras); it's a baffling problem (Taras); it doesn't make sense (Taras); the mind cannot grasp it; it's unfathomable (Taras); it's impossible to comprehend (Taras); it's impossible to explain (Taras); it's beyond human understanding (Taras); this is incredible (Taras); there's just no explaining how (what, etc Taras); it's absolutely unfathomable (Taras); it's beyond all understanding (Taras); it's beyond all belief (Taras); it defies understanding (Taras); it is mind-boggling (Taras); it's mind-boggling (Taras); mind reels (otlichnica_po_jizni); it's inconceivable (Taras); beyond all comprehension

Gruzovik, 非正式的

it is inconceivable; it is beyond human understanding

不赞成

beyond my comprehension ("Everything is known, Colonel Walter," said Sherlock Holmes. "How an English gentleman could behave in such a manner is beyond my comprehension." (Sir Arthur Conan Doyle) ART Vancouver)

修辞格

it boggles the mind (Leonid Dzhepko); it is beyond me (Leonid Dzhepko); it boggles the mind (- Вот он всегда так, – ворчливо, но не без гордости проговорила Жена. – Сколько он этих тостов знает – ну просто уму непостижимо! – "He's always like that,' said the Wife, not without a note of pride in her voice despite its peevishness, 'there's no end to the toasts he has up his sleeve – it's enough to make the mind boggle!" Taras)

谚语

it passes understanding (used as pred.)

非正式的

it's inconceivable; it passes all understanding; can't wrap one's head around something (I just can't wrap my head around how he managed to get himself into this mess! Просто уму непостижимо как он сумел так вляпаться! russoturisto)

amazing! (Abysslooker)

уму непостижимый

Игорь Миг

jaw-dropping; end-of-the-world; undreamed-of; undreamt-of; dizzying

修辞

mind-boggling (CNN Alex_Odeychuk)

修辞格

mind-bending (Scientists achieve mind-bending 'quantum time flip'