"Felina" is an anagram for "finale", and is also the feminine version of the word "feline" in Italian and Spanish.
The title is a reference to the 1959 song "El Paso" by Western music artist Marty Robbins. The song concerns an unnamed cowboy who falls in love with a woman named Felina, gets shot by his enemies, and dies in her arms. The song plays in Walt's stolen car in New Hampshire, and is later hummed by Walt as he assembles the M60.
Walt's birth year matches the year which Marty Robbins song "El Paso" was released.