The interpretation of the message conveyed by Thomas Nast's political cartoon depicting Boss Tweed laughing during his arrest, observed by the Shadow of Justice, is as follows.
Boss Tweed epitomized the corrupt activities of interest groups within United States politics during the mid-19th century.
As a prominent figure in the Democratic Party's political machine known as "Tammany Hall" in New York City, he was also a businessman with various economic interests and political motivations that involved bribing lawmakers for personal and business gain.
In the cartoon, Thomas Nast illustrates Tweed’s influence in New York through the text: "Sheriff Brennan merely nodded to Mr. Tweed, bade him 'Goode-day,' and laying his hand tenderly on his shoulder, said laughing, 'You're my man!' This represents Tweed's belief that he could manipulate everyone, including the legal system.