Abreast

From Middle English a ("on") + brest ("breast"), in sense â€œbreasts (chests) in line, side-by-side and exactly equally advancedâ€;

Online Etymology Dictionary

roughly â€œbreast-by-breastâ€.

Full definition of abreast

SOED5|page=8

2012, July 15, Richard Williams, Tour de France 2012: Carpet tacks cannot force Bradley Wiggins off track, On Sunday afternoon it was as dark as night, with barely room for two riders abreast on a gradient that touches 20%.
Thomas Babington Macaulay, Two men could hardly walk abreast.

(figurative) Alongside; parallel to. First attested in the mid 17^th century.
Informed, well-informed, familiar, acquainted. First attested in the mid 17^th century.
Up to a certain level or line; equally advanced; as, to keep abreast of with the present state of science. First attested in the mid 17^th century.
(nautical) Side by side; also, opposite; over against; on a line with the vessel's beam. First attested in the late 17^th century.
(obsolete): At the same time; simultaneously

(nautical) Abreast is followed by the word of.

(alongside) Abreast is followed by with or of.

(informed) Abreast is followed by with or of.

(up to a certain level) Abreast is followed by with or of.

SOED5|page=8

(figurative) Alongside; parallel to. First attested in the mid 17^th century.
Informed, well-informed, familiar, acquainted. First attested in the mid 17^th century.
Up to a certain level or line; equally advanced; as, to keep abreast of with the present state of science. First attested in the mid 17^th century.
(nautical) Side by side; also, opposite; over against; on a line with the vessel's beam. First attested in the late 17^th century.

MW3 1976|page=5