very beginning of a book
Rare literary expression referring specifically to the opening lines or first page of a book. Not used in everyday conversation.
The very first line of the book captures the reader's heart.
冒頭 is the common word for 'beginning' of a text or speech, while 開巻劈頭 is a rare, book-specific literary term.
書き出し is a general term for the opening of a written work, much more common than 開巻劈頭.
A four-character compound of classical origin. 開巻 means 'opening a book', and 劈頭 means 'at the very start'. The exact historical derivation is uncertain, but it is a set phrase in literary Chinese and Japanese.