Named after Jean-Marc Bosman, whose 1995 European Court of Justice case established that players could move freely at the end of their contracts. Before Bosman, clubs could demand a fee even after a contract expired. The ruling fundamentally changed football economics and gave players unprecedented power. 'Going on a Bosman' means leaving for free when your contract ends.
"He's running down his contract. They'll lose him on a Bosman in the summer."