An Etymological Dictionary of the German Language/Brust
Brust, f., ‘breast, chest, pap,’ from the equiv. MidHG. brust, OHG. brust, f.; it corresponds to Goth. brusts, a plur. noun (conson. stem), f., Du. and LG. borst. In the other OTeut. dialects the words corresponding exactly to Goth. brusts are wanting; they have a peculiar neut. form: AS. breóst, E. breast, OIc. brjóst, OSax. breost, which are related by gradation to HG. Brust. This term for breast is restricted to the Teut. languages (including OIr. 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.