The noun solidarity  originated in the early 19th century from the French word solidarité (coined around 1765) by the Encyclopédie,  it initially referred to a legal or social obligation where individuals are tied together, or "interdependent" (solidaire).  It is rooted in the Latin solidus ("whole," "firm," "solid") and solidum ("whole sum"), highlighting the conceptual connection to unity, strength, and shared responsibility (a debt for which each party is fully responsible).  Evolution of Meaning: 16th-18th Century: Legal context (obligation solidaire)—connected responsibility.  19th Century: Social/political context—shared unity, communion of interests.  Before becoming a political term, the concept existed in the 1804 Napoleonic Code as des obligations solidaires, referring to joint liability in law.  It gained prominence in the 1840s through French utopian socialists like Pierre Leroux, who used the concept to emphasize social interconnectedness.  20th Century: Politicized to mean camaraderie and collective action, notably with the Polish trade union Solidarność in 1980.  The core meaning remains a bond of unity or agreement, often for a common cause, maintaining the essence of being "solid" or "entire" together. 

According to the Oxford English Dictionary (OED), the earliest known use of the noun solidarity in English is from 1841, often noted in the writing of H. Doherty.   Some studies indicate that by 1848, it was recognized as a word of French origin whose "naturalisation" in English was considered desirable. 

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SOLIDARITY Definition & Meaning - Merriam-Webster

SOLIDARITY Synonyms: 63 Similar and Opposite Words | Merriam-Webster Thesaurus

SOLIDARISM Definition & Meaning - Merriam-Webster