Unsound doctrine is the opposite of sound doctrine. It refers to false teachings.
A failure to endure sound doctrine logically gives rise to unsound doctrine, and unsound doctrine in the professing church has historically led to critical perversions of God’s grace in Christ and the gospel itself.
– Source: http://gomarus.wordpress.com/2006/08/14/why-make-issues-over-doctrine: Why make issues over doctrine? Morning Coffee with Gomarus [No longer online]