From 返信(へんしん) (henshin) + 待(ま)ち (machi, “連(れん)用(よう)形(けい) (ren'yōkei, “stem or continuative form”) of the verb 待(ま)つ (matsu).”).
返(へん)信(しん)待(ま)ち • (henshin-machi)