On the way to a place; during the act of going somewhere. Usually used adverbially as '行きしなに'. Similar in meaning to '行きがけ'.
行きしなにコンビニで飲み物を買ってきてください。
Please buy a drink at the convenience store on your way.
行きしなに財布を忘れたことに気づいた。
I realized I had forgotten my wallet on the way.
仕事へ行きしなに、郵便局に寄った。
On my way to work, I stopped by the post office.