Dot you ess! It's the first two letters of USA.

.us is the country-specific top-level domain (TLD) for the United States. Other well-known country-specific TLDs are .au for Australia, .ca for Canada, .cn for China, .de for Germany, .fr for France, .jp for Japan, .ru for Russia, and .uk for the United Kingdom. For a list of country-specific top-level domains, see http://www.iana.org/cctld/cctld-whois.htm.

Until recently, .us was mostly used by government entities, for example, http://www.state.ut.us (State of Utah), http://www.alpine.k12.ut.us (Alpine School District), http://www.ci.west-valley.ut.us (West Valley City), etc. Use of the .us country-specific TLD is restricted to United States citizens, businesses, government entitites and other organizations.