In order to convey the feeling of ‘what the heck’, with any ‘how’, ‘what’, ‘where’ or ‘why’ type question, the 副詞（ふくし） (adverb) 一体（いったい）
will be used in Japanese. いったい appears frequently both with, and without kanji.
いったい will usually be seen at the beginning of sentences, before some statement about what the speaker finds unbelievable.
What the heck happened here?
What is the meaning of this?
The literal meaning of いったい is ‘one body’, or ‘one form’. In this way, when paired with questions, いったい can be thought of meaning ‘in a way that can be understood, what is (A)’, or ‘in a way that makes sense, what is (A)’.
Explain to me in a way that makes sense, why something like this happened?
In a way that I can understand, why did you think of dating someone like him?
Like the ‘one body’, or ‘one form’ meaning mentioned above, 一体（いったい）
is sometimes used to mean ‘in general’, or ‘generally’ when seen as 一体（いったい）に. However, this use is slowly becoming less common, partly due to how often 一体（いったい）
is used simply to express surprise or shock.