The phrase Noerr Immunity is a result of the Eastern Railroad Presidents Conference v. Noerr Motor Freight Co. case that was heard by the Supreme Court in 1961. At the time, railroads and the truckers were engaged in a battle over transporting freight and in an effort to thwart the truckers, the railroads came together to lobby the government to ban the tranport of heavy freight on the roads. The truckers claimed that by coming together to lobby, the railroads who normally competed with one another, were violating section 1 of the Sherman Act by forming a conspiracy against the the truckers.
The railroad agreement was pretty much exactly what the Sherman Act was designed to take down, only here the Court found that because they were lobbying the government, their actions were protected. In essence. the Court had to narrow the construction of the Sherman Act so as not to find it unconstitutional since our 'right to petition' is secured by the First Amendment.
This same line of reasoning would later be applied in the Parker case which extended immunity to state action.