‘何を隠そう’ (nani o kakusou) is an idiomatic expression that literally means ‘What would I conceal? (I won’t any)’. This expression is used when you want to emphasise information that you are about to share with others (similar to “in fact” in English). For instance, The sentence ‘何を隠そう、私が彼と彼女を結びつけたのだ’ (nani o kakusou, watashi ga kare to kanojo wo musubituketa noda) is translated as ‘What would I conceal? I brought him and her together’ (≒ ‘You know what? It’s me who brought him and her together.’).
As in the example above, this expression is often employed when you reveal something about yourself. Although mostly used in writing, the idiom contains rather a casual and playful tone. It is also used in Anime, and therefore if you used this phrase in speech, chances are you sound like a weird anime otaku.
This expression is short for ‘何を隠すだろうか (Nani o kakusu darouka; What would I conceal?)’, and it is kind of a rhetorical question which is intended to be followed by ‘何も隠さないだろう (Nani mo kakusanai darou; I won’t conceal anything)’. Here, 隠す (kakusu) means ‘conceal’ or ‘hide’.
Nani o kakusou (What would I conceal/You know what?) It’s me who stole the treasure from this island!
A lot of people end up procrastinating thier work, and nani o kakusou (What would I conceal/You know what)? I am also one of them.
Synonyms in English: You know what?
Related words and phrases: 隠す (hide, conceal)