expression, noun which may take the genitive case particle 'no'
breathtaking; stunning; awe-inspiring
Used as a prenominal modifier (～の) to describe something so beautiful, impressive, or shocking that it takes one's breath away.
It was a breathtakingly beautiful view.
Her performance was stunningly wonderful.
Standard kanji spelling using 呑む, which is common in this expression.
Variant using 飲む; both spellings are used, but 呑む is more common in this fixed phrase.
From 息 (breath) + を + 呑む (to swallow, hold) + ほど (extent). Literally 'to the extent of swallowing one's breath', used to describe something so striking it makes one hold their breath.