immediately; instantly; without a moment's delay
Rare variant of 間髪を入れず. Used in literary or emphatic contexts to mean 'without even a hair's breadth of pause'. Learners should prefer 間髪を入れず.
See also: 間髪を入れず
They immediately moved to counterattack without a moment's delay.
The standard, common expression with the same meaning. 間一髪を入れず is a rare variant that replaces 髪 with 一髪, possibly for emphasis.
A reshaping of 間髪を入れず under the influence of 間一髪. In the older expression, 間髪 means an interval as narrow as a hair.