1.
just when, right when, at the very moment
A point in time just as something was about to happen or had just started. Typically followed by an unexpected or inconvenient event.
Usage
Used in the pattern ~した矢先に or ~しようとした矢先に. The event that follows is almost always unexpected, inconvenient, or unfortunate. This creates a sense of bad timing or irony.
Etymology
矢先 literally means "arrowhead" (矢 = arrow, 先 = tip). The metaphor suggests the precise point where the arrow is aimed — hence, the exact moment.