What is Sinistrisme?

Legal Definition
Sinistrisme is a neologism invented by Albert Thibaudet in Les idées politiques de la France (1932). He referred to the progressive substitution of left wing parties by new, more radical parties, which in turn pushed each party towards the center (the Radical Party being replaced by the French Section of the Workers' International (SFIO, a socialist party), the Socialists by the French Communist Party, etc.). This historical movement (seen by Thibaudet as a historical necessity) thus explained, according to Thibaudet, the tendency of the right-wing, in particular in France, to disavow the label droite (right), long associated in French history with monarchism, in favour of left.

Hence, those who rallied to the Republic in 1893 still adopted the denomination Droite constitutionnelle or républicaine (Constitutional or Republican Right), but replaced it in 1899 by Action libérale, under which they went to the 1902 elections. An explicit right wing group resurged in 1910, gathering the last nostalgics of the monarchy. According to historian René Rémond, since 1924 the term "right wing" vanished from the parliamentary group's glossary. Deputies from the Democratic Republican Alliance (ARD), which was the main center-right parliamentary formation under the Third Republic, sat in the parliamentary group of "Left-wing Republicans". Following the Liberation, the Rally of Left Republicans (RGR) gathered conservative deputies, mostly from the Radical-Socialist Party (and opponents of Pierre Mendès-France as well as Independent Radicals who had left the Radical Party in 1928, opposed to its alliance with the left-wing during the Cartel des gauches), and from the UDSR.

René Rémond remarked that "at the 1974 presidential election, only one candidate declared himself as belonging to the right-wing: Jean-Marie Le Pen; in 1981, no one." Conservative (which had been the name of an ultra-royalist review in 1818-1820) was a synonym of "right wing" often used under the Third Republic, in particular by the Bloc national Chamber. Independents, used in the 1920s for deputies close to the Action française royalist movement, was later used by less reactionary politicians.
-- Wikipedia