expression, godan verb (-su)
push out one's chest; offer one's chest
Sumo term: a senior wrestler lets a junior push against his chest during practice to help the junior train. Literally 'to put out one's chest'.
親方は若い力士に胸を出して、ぶつかり稽古をつけた。
The stablemaster offered his chest to the young wrestler and gave him butsukari-geiko (chest-pushing practice).
Compound of 胸 (chest) + を (object marker) + 出す (to put out). The sumo usage is a specialized extension of the literal meaning.