Picture a row of people walking shoulder to shoulder down a lane. Their chests are all on the same line, and everyone faces the same way. Nobody is in front and nobody has dropped back. That single picture — level, side by side, moving together — can be taken as the thread behind every use of abreast, whether it is bikes on a road, a boat coming up beside an island, or a person keeping level with the news.
From the word’s history
The source records abreast as Middle English abrest, formed from a- (on, at) plus breast — roughly breast-by-breast, chests in line and equally advanced. That literal shape sits right on top of the modern picture of a level row.
資料記的是：abreast 來自中古英文 abrest，由 a-（在…上）加 breast（胸口）組成，字面大致是「胸口對胸口」——一排胸口對齊、前進得一樣多。這個字面正好跟今天「齊頭並排」的畫面疊在一起。
Wiktionary, "abreast"
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