The skirt steak is traditionally used for fajita meat. Similar to the flank steak, it is a very lean cut with long muscle fibres that need to be cut thinly across the grain.
The skirt is considered a slightly better cut than flank because it takes up the flavour of a marinade better and gives a more tender finished beef. It is not only excellent in fajitas, but also in sandwiches or as a salad topping.
Marinate for at least a half hour, or overnight if possible. Cook 3-4 minutes per side and cut thinly across the grain.