The meanings of words often change over time. According to Merriam-Webster:
Definition of fajita
: a marinated strip usually of beef or chicken grilled or broiled and served usually with a flour tortilla and various savory fillings —usually used in plural
Origin and Etymology of fajita
American Spanish, diminutive of Spanish "faja" - sash, belt, probably from Catalan faixa, from Latin fascia band
https://www.merriam-webster.com/dictionary/fajita
But my favorite food history page says this: "Fajita. A Tex-Mex dish made from marinated, grilled skirt steak...served in a wheat tortilla. The word derives from the Spanish faja, for "girdle" or "strip" and describes the cut of meat itself. There has been much conjecture as to the fajita's origins, though none has been documented. Grilling skirt steak over mesquite coals would be characteristic of Texas cooking since the days when beef became a dominant meat in the American diet. But the word "fajita" did not appear in print until 1975."
Food Timeline--Mexican and TexMex food history
So that used to be true