Roman debtors who were liable in solidum owed the full amount together.
In Roman law, the phrase in solidum described a form of joint liability. When multiple debtors owed a sum in solidum, each was responsible for the entire debt, not just a portion.1 The obligation bound them together. If one could not pay, the others absorbed the full burden.
The French solidarité emerged in the 1840s, initially as a legal and then a philosophical term. Auguste Comte used it to describe the mutual dependence of individuals within society. Émile Durkheim later built an entire theory of social cohesion around it, distinguishing between mechanical solidarity in traditional societies and organic solidarity in industrial ones.2
In English, solidarity took on political force in the labor movement. Trade unions adopted it as a principle: an injury to one is an injury to all. The word migrated from contract law to the picket line.
In 1980, the name became globally famous when Polish workers at the Gdańsk Shipyard formed Solidarność, an independent trade union that challenged Communist rule. Led by electrician Lech Wałęsa, it grew to nearly ten million members within a year.3
Solidarność was the first independent labor union in a Soviet-bloc country. Its emergence contributed to the broader unraveling of Communist governments across Eastern Europe throughout the 1980s. Wałęsa received the Nobel Peace Prize in 1983 and became president of Poland in 1990.4
Durkheim had argued in 1893 that industrial class society replaced the solidarity of shared identity with a solidarity of interdependence, where each person's specialized role bound them to everyone else.5 A word born in Roman debt law, refined by French sociology, and forged in a Polish shipyard.