Legalism means “law-ism.”  From the term itself it simply means that the law is exalted.  It does not say how or why one does so.

A person who believed that his devouring of widow’s houses was OK with God because he tithed everything he bought as well as earned as a sign of God’s grace on his life (i.e. he thanked God as the source of his moral ability to tithe and devour widow’s houses) would be a “legalist”–he would define himself as favored by God on the evidentiary basis of keeping some rules that he associated with the Law.

If Jesus met such a person, he would condemn him as lawless, tell him to repent, warn him of impending judgment, and exhort him to actually obey the Law.

Jesus’ exhortation to obey the law would not be an exhortation to become sinless.  It would not be an attempt to get the person to stop despising the Law.  Jesus didn’t tell people to love their wives because he wanted them to be convicted that they lost their tempers and didn’t love them sinlessly.  He told them to love Samaritans whom they had convinced themselves they were obeying the law by hating.  They wanted to hate Samaritans and boasted that they did so better than other people.  The dynamics of striving to be good enough are completely absent from the Gospels.

When people refused to believe Jesus’ interpretation they were trusting in themselves that they were righteous rather than trusting in Jesus that their righteousness was odious sin.  It was not odious because it failed to measure up.  It was odious because it was going in an entirely wrong direction.