one-sided; committed to one side or party; prejudiced
I should tell you in advance that my opinion is a partisan one, since I am a committed supporter of the Liberal party.
pars partis (part) (Latin)
parti (think of a political party) +san (son): so a father who is favoring his son's political party, instead of a good party. A partisan says, 'my party is my SON, my daughter, my mom & my dad'