Perfect explanation, I looked this sentence up few days ago, and I've found out it means: walk next to each other facing the same way.. It's a pretty good metaphor."Abreast" comes from "breast" meaning petto, not seno. If people walk "two abreast" it means that they are not in single file but their breasts (petti!!) are at the same point, so no one is behind. To keep someone abreast means not to leave them behind. Hence the metaphoric meaning.