〜矢先(に)
just when (about to), just as
Verb volitional + とした矢先に · Verb た-form + 矢先に
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矢先に means "just as", right at the moment someone was about to do something, or had just done it, when something unexpected happened: 出かけようとした矢先に、電話が鳴った, "just as I was about to go out, the phone rang".
The two common shapes are 〜ようとした矢先に (about to) and 〜た矢先に (had just). The event in the second half is almost always unwelcome: an accident, bad news, an interruption.
矢先 is literally "the tip of an arrow", the moment of release. Compare 最中に (N3), "in the middle of doing", and ところに (N2), "just then", which is more neutral.
Examples
出かけようとした矢先に、電話が鳴った。
Just as I was about to go out, the phone rang.
家を買った矢先に、転勤を命じられた。
Just after we'd bought a house, I was ordered to transfer.
禁煙を決めた矢先に、たばこをもらった。
Just when I'd decided to quit smoking, someone gave me cigarettes.
注意しようと思っていた矢先に、事故が起きた。
Just as I'd been meaning to warn them, there was an accident.
仕事に慣れてきた矢先、病気で倒れた。
Just as he was getting used to the job, he fell ill.