From the OED: Having a purposed tendency; composed or written with such a tendency or aim.
1900 T. DAVIDSON Hist. Educ. I. iv. 70 Xenophon's Cyropædia..is a mere edifying, tendentious romance, intended to recommend to the Athenians the Spartan type of education. 1905 Times, Lit. Suppl. 28 July 239/2 He [Zimmer, in ‘Die Keltische Kirche’] thinks that the legend of St. Patrick was tendencious, springing up to support a special ecclesiastical thesis. 1909 C. LOWE in Contemp. Rev. July 42 A false and tendencious account of what had taken place.Hmmmm... the OED isn't exactly clear on the meaning. Merriam-Webster is better when it offers biased as a synonym.