Shariia Is a Bugaboo word

Shariia is a bugaboo word tossed around a lot. Many of the people who tosses this word are ignorant people. So what is sharria? Actually it is undefined word and Moslem countries had adopted differently. Shariia is not rigid. So what is it? if shariia says that you should not marry your sister would you say shariia is tyranny and I am going to marry my sister or aunt. Would you marry your mother?
for many Shariia is a Bugaboo word to scare people!
So what is sharria and is it cast in iron? or very adaptable to time, society and countries?