expression, godan verb (-su)
allow a less skilled person to practice with one; be a practice partner for a weaker player
Primarily used in sumo, where a higher-ranked wrestler lets a lower-ranked wrestler practice against him. Can be extended to other sports or skills where a stronger person helps a weaker one train.
See also: 胸を借りる
横綱が若い力士に胸を貸した。
The yokozuna allowed the young wrestler to practice with him.
先輩が胸を貸してくれたおかげで、だいぶ上達した。
Thanks to my senior letting me practice with him, I improved a lot.