An Etymological Dictionary of the German Language/Annotated/Brust
Brust, feminine, ‘breast, chest, pap,’ from the equivalent Middle High German brust, Old High German brust, feminine; it corresponds to Gothic brusts, a plural noun (conson. stem), feminine, Dutch and Low German borst. In the other Old Teutonic dialects the words corresponding exactly to Gothic brusts are wanting; they have a peculiar neuter form: Anglo-Saxon breóst, English breast, Old Icelandic brjóst, Old Saxon breost, which are related by gradation to High German Brust. This term for breast is restricted to the Teutonic languages (including Old Irish bruinne, ‘breast’?), the individual members of the Aryan group differing in this instance from each other, while other parts of the body (see Bug) are designated by names common to all of them. Of the approximate primary meaning of Brust, or rather of the idea underlying the word, we know nothing; the only probable fact is that the primitive stem was originally declined in the dual, or rather in the plural.