Pushing forward with the chest, or moving against resistance; also, the action of coming to grips with something.
From 'breast' (verb form, to push) + '-ing' (present participle). The verb 'breast' comes from Old English and means to face or push against.
When Shakespeare wrote about 'breasting the waves,' he was using one word to describe the whole physical act of plowing forward against resistance—it's poetic, exact, and describes how ships and swimmers alike move through water.
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