foul, rejected meat: consecrated or forbidden flesh that has become desecrated and unfit to eat or offer
Piggul is a rare, technical word from Israel's priestly vocabulary, turning up only four times in the whole Bible. At its core it names foul, rejected flesh: meat that has become desecrated and unfit, something once holy or edible that has crossed a line and turned taboo. Its origin is genuinely unknown, so scholars reconstruct its sense from the four places it appears rather than from any secure root.
In Leviticus the word is tied to one specific rule. The meat of a fellowship offering could be eaten on the day it was sacrificed and the next day; whatever was left by the third day had to be burned. If someone ate it on the third day instead, the meat was piggul: the offering was cancelled, counted as worthless to the worshipper, and the person who ate it bore the guilt. Consecrated flesh, in other words, had a shelf life of holiness, and eating it past that point did not merely spoil it but desecrated it.
The two prophets use the word more loosely. Ezekiel, himself a priest, lists "foul flesh" beside meat from animals that died on their own (נְבֵלָה) and animals torn by beasts (טְרֵפָה), as a class of disgusting, forbidden food he had never eaten. Isaiah uses a plural, "foul things," for what fills the vessels of people who practice secret, illicit rites. Scholars read these not as a different word but as the same term stretched past the narrow sacrificial rule to mean foul, abhorrent food of any kind.
Most modern scholars take the offense in Leviticus to be the actual eating of holy meat past its time, which leaves the meat desecrated and foul. A famous later Jewish legal tradition instead defined piggul as a sacrifice ruined by the priest's improper intention, formed silently during the ritual, rather than by anyone eating it late. That reading became central to rabbinic law, but it is a later development, and the tradition itself admits it departs from the plain sense of the verses.