What does abreast mean?
The definition of abreast
A*breast”, adv.
Etym: [Pref. a- + breast.]

1 Side by side, with breasts in a line; as, “Two men could hardly walk abreast.” Macaulay.
2 (Naut.)
Side by side; also, opposite; over against; on a line with the vessel’s beam; — with of.
3 Up to a certain level or line; equally advanced; as, to keep abreast of [or with] the present state of science.
4 At the same time; simultaneously. [Obs.] Abreast therewith began a convocation. Fuller.