late 14c., "pleasing to the sight, fair," from Latin speciosus "good-looking, beautiful, fair," also "showy, pretended, plausible, specious," from species "appearance, form, figure, beauty" (see species). Meaning "seemingly desirable, reasonable or probable, but not really so; superficially fair, just, or correct" in English is first recorded 1610s. Related: Speciously; speciosity; speciousness.
Specious is superficially fair, just, or correct, appearing well at first view but easily proved unsound. Plausible is applied to that which pleases the ear or the superficial judgment, but will not bear severe examination. [Century Dictionary, 1895]
1888, "hard work," Australian slang, of unknown origin, perhaps from Lincolnshire dialect. Adjectival meaning "honest, genuine" is attested from 1916. Phrase fair dinkum "fair play" is attested by 1894.