Lou Ferrigno Exercises His Right to Threaten to Sue His Brother

Lou Ferrigno is angry at his brother (and you won't like him when he's angry) but instead of turning green ... he's skipping right to threatening a lawsuit over his bro's exercise equipment business.

Lou and his brother Andrew had a legal skirmish a few years back and eventually reached a settlement ... which Lou says limits Andrew to using the phrase "Andrew Ferrigno equipment." He also has to make it clear his biz has no affiliation with Lou's companies and websites.

Lou claims Andrew has violated the order because he's using the word "exercise" in the name of his store ... and insists the disclaimer is nowhere to be found on Andrew's website or social media.

Lou's lawyers sent a letter threatening to sue ... and Andrew tells us since then he's met all of his brother's demands. Well, except for removing the word "exercise" from his store's name.

Andrew tells TMZ, "My brother has money, so why does he need to suck me dry? I am so bewildered by the fact that an actor of his stature would stoop so low as to sue his own brother twice."