Shi'ism can refer to the second largest sect of Islam, which is practiced and followed by the Shia (the followers of Ali.) In its more precise meaning it refers more to the identity politics of the Shi'a and the form of activism and empowerment that the historically persecuted and neglected people have embraced since the 1970s (like Musa Sadr's Movement of the Deprived in southern LEbanon). It is sometimes used in a derogatory manner by fearful Sunnis who look down on the Shi'a , and deride their ambitions for political representation as "expansionist" and a threat to the Sunni umma (community). Shi'ism can also be viewed as an expandable formula for political activism (according to As'ad AbuKhalil, Shi'a groups like Hezbollah follow a Leninist organizational scheme) that could inspire other movements of the persecuted and deprived to fight for their rights. Shi'ism has further solidified the bond between the disparate pockets of Shi'ites throughout the Muslim world (Iran, Iraq, Lebanon, Bahrain, Saudi Arabia, etc.), creating something of a pan-Shia identity.
Pro-western Sunni Arab puppet dictator Nr. 1: Man, this revived brand of Shi'ism is a real threat to us. These Shi'ites are getting their shit together and our Sunni populations are soon going to start making demands on us to provide for them like the Hezbollah in southern Lebanon has for their people. We can't let Shi'ism take us down, oh Guardian of the two holy sites.
Pro-western Sunni Arab puppet dictator Nr. 2: Yeah, dude, I know what you mean. I've been feeling the heat from our own Shi'ites in the eastern regions of my kingdom. I just slaughtered a few thousand of them to warn the others to knock it off. But if Khomeini's Shi'ism starts spreading, we are going to be f**ked. We might as well make good with the Zionists so that they can help us slaughter them.