also: かんはつをいれず
expression
immediately; instantly; without a moment's delay
Literally 'without putting a hair's breadth in between'; used for actions that follow another with no gap in time. Often used in narrative or formal contexts.
彼は質問に間髪を入れず答えた。
He answered the question without a moment's delay.
間髪を入れず反論が返ってきた。
A rebuttal came back instantly.
Alternate kanji for いれる; less common.
From the idiom 間髪を容れず (かんぱつをいれず), literally 'not even inserting a hair's breadth'. 間髪 (かんぱつ) means the space of a single hair, and 容れず is the negative of 容れる (いれる, 'to insert'). The phrase describes an action so immediate that not even a hair could be placed between events.