The first FAST pack (that I know of) was a conformal fuel tank with hardpoints for ordinance designed for the F-15 (primarily the dual-role E variant, the Strike Eagle).
The acronym stands for Fuel And Sensor Tactical.
[Edit: What EXO said]
Fuel and Sensor Tactical packs were the brainchild of an experienced engineer working in the ‘Advanced F-15 Design engineering’ team, called Frank Laacke. They consisted of aerodynamically shaped fuel tanks, which slotted in flush between the lower surface of the wing and the side of the engine intakes. Laacke explained, “We started out with the F-15 being a supersonic interceptor. With external fuel tanks on you lose a lot of acceleration capability, we therefore needed to come up with another way of carrying fuel. We played around with a number of ideas, but finally settled on the concept of FAST packs. They provided reduced supersonic drag and a very small amount of subsonic drag, they also had very little effect on stability. As they evolved the Air Force lost interest in extending the range of the jet as an interceptor and we started to look at FAST packs for increasing ferry range and carrying bombs.
F-15E (with FAST packs)
F-15 (without FAST packs)