We've already had his origin in Texas Chainsaw Massacre the beginning:
In 1939, a woman dies while giving birth in a slaughterhouse, and the manager attempts to dispose of the baby by leaving him in a dumpster. A young woman, Luda Mae Hewitt, finds the child, takes him back to the Hewitt residence, names him Thomas, and decides to raise him as her own. Thirty years later, Thomas, also known as Tommy, is working in the slaughterhouse under the same manager. When the plant is shut down by the health department, he refuses to leave until the manager forces him to. Later, Tommy returns, murders the manager, and then finds a chainsaw. Luda Mae's son, Charlie Hewitt, kills the sheriff "Hoyt" to prevent him from arresting Tommy. He then assumes his identity and takes his body back home to use for stew meat.