"a gypsy of society; person (especially an artist) who lives a free and somewhat dissipated life, despising conventionalities and having little regard for social standards," 1848, from a transferred sense of French bohemién "a Bohemian; a Gypsy," from the country name (see Bohemia). The Middle English word for "a resident or native of Bohemia" was Bemener.
The French used bohemién since 15c. to also mean "Gypsy." The Roma were wrongly believed to have come from there, perhaps because their first appearance in Western Europe may have been immediately from Bohemia, or because they were confused with the 15c. Bohemian Hussite heretics, who were driven from their country about that time.
The transferred sense, in reference to unconventional living, is attested in French by 1834 and was popularized by Henri Murger's stories from the late 1840s later collected as "Scenes de la Vie de Boheme" (the basis of Puccini's "La Bohème"). It appears in English 1848 in Thackary's "Vanity Fair."
The term 'Bohemian' has come to be very commonly accepted in our day as the description of a certain kind of literary gipsey, no matter in what language he speaks, or what city he inhabits .... A Bohemian is simply an artist or littérateur who, consciously or unconsciously, secedes from conventionality in life and in art. [Westminster Review, 1862]
Hence also the adjective, "unconventional, free from social restraints" (1848).