Side by side; next to each other in a line; or informed about the latest news or developments.
From 'a-' (to/at) and 'breast' (chest/front). Literally means 'beside the breast' or 'side by side.' First meaning referred to positioning, later extended metaphorically.
The phrase 'keep abreast of' (staying informed) comes from the military meaning—soldiers marching 'abreast' are shoulder-to-shoulder, so 'abreast of news' means you're right alongside the latest information, not falling behind.
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