Full Eph 6:14, Isa 59:17. The Roman lorica (breastplate: banded segmentata or mail hamata, see [section 1]). OT echo: Isa 59:17, "he put on righteousness as a breastplate" (ASV). The armor Paul tells the believer to wear is the armor YHWH already wore.
Matthew Henry ties the image directly to the forensic reading: "The righteousness of Christ imputed to us is our breast-plate against the arrows of divine wrath.", Matthew Henry, Commentary on the Whole Bible (1710), on Eph 6:14
Contested, honestly held both ways: is the breastplate imputed or practical righteousness? The Reformed reading (Sproul, Piper, Packer) holds it as Christ's righteousness reckoned to the believer's account, received by faith alone, the breastplate is put on once, by grace, not assembled by works.
The Wesleyan-holiness reading (Wesley) and the Reformed-Puritan reading (Ryle, Owen) both press that a merely legal breastplate does not stop an arrow, the righteousness worn into battle must be lived, progressively, or the soldier goes in exposed regardless of standing. Where the readings meet, held together without flattening either: justification is imputed and forensic, Christ's righteousness reckoned, received by faith, complete and unrepeatable, while sanctification is positional and progressive, worked out day by day.
The breastplate is both: the armor already given, and the armor still being put on.