In order to better understand why Texas is called this way you have to learn a little history: after Columbus discovered the Americas the first europeans that arrived in the region where the Spanish conquistadors (conquerors) in the XVIth century, finding it populated by various Native American tribes. During the period from 1519 to 1848, all or parts of the current state of Texas were cl aimed by six countries: France, Spain, Mexico, the Republic of Texas, the United States of America and the Confederate States of America in 1861–65.
Texas is from the Caddo Indian word teyshas (meaning friends or allies). The Caddo Indian tribe Hasinai originally lived along the Red River in Louisiana, and then began to migrate northwest and south. Spanish explorers took this to be a tribal name, translated and recorded as Teyas or Tejas. Later it came, eventually, to mean an area north of the Rio Grande and east of New Mexico. The alliance concept is also incorporated into the state motto, which is simply Friendship. Texas's nickname is The Lone Star State, named after the one star on its flag.
The first European base was established in 1682, when a French colony was established at Fort Saint Louis, near Matagorda Bay. The colony was killed off after only three years, but its presence motivated Spanish authorities to begin activity: in the next years several missions were established in East Texas, but they were all abandoned in 1691. 20 years later Spanish authorities again attempted to colonize Texas, the following (almost) 2 centuries Spain established numerous villages, presidios, and missions in the province. A small number of Spanish settlers arrived in addition to missionaries and soldiers. When Mexico won its independence from Spain in 1821, Mexican Texas was part of the new nation.
Spanish control of Texas was followed by Mexican control, it can be difficult to separate the Spanish and Mexican influences on the region. The most obvious legacy is the language: every major river in modern Texas, except the Red River, has a Spanish or Anglicized name, as do 42 of the state's 254 counties. Numerous towns also bear Spanish names.
On February 1845 the U.S. Congress passed a bill that would authorize the United States to annex the Republic of Texas. The legislation set the date for annexation for December 29 of the same year. Texas received its flag in 1845 when it became part of the United States. The same as the U.S. flag, the red stands for courage, the blue is for loyalty and the white is for strength.