Boyd’s Bible Dictionary:

The hog was pronounced unclean
(Lev. 11:7; Deut. 14:8). Priests and Arabians abstained from the meat for dietetic reasons. Swine-keeping a degrading business (Luke 15:15); yet swine were kept (Matt. 8:32). To cast “pearls before swine” was to waste truth on those who despised it (Matt. 7:6).

Concise Bible Dictionary:

One of the animals classed among the unclean, and which is supposed to have been held in abhorrence as food by the Jews. The prophet Isaiah, however, charges them with eating swine’s flesh; and their apostasy was such that he says when they offered an oblation, it was as if they had offered swine’s blood: their heartless profession was abhorrent to God (Isa. 65:4; Isa. 66:3,17). It is not recorded whether the Gadarenes were Jews or Gentiles, who lost their swine by the demons’ possession of them (Matt. 8:32; Mark 5:13). The swine are typical of the most defiled and degraded, to whom apostates are compared, and before whom holy things should not be cast (Matt. 7:6; 2 Pet. 2:22). The prodigal had reached the lowest point of degradation when he would fain have satisfied his hunger with the swine’s food (Luke 15:16).

From Manners and Customs of the Bible:

Luke 15:15. He sent him into his fields to feed swine.
This was considered one of the most degrading employments, not only by the Jews, but by other nations. Among the Egyptians, for example, the swineherd was completely shut out from society. The Saviour makes use of this antipathy to illustrate the depth of misery to which the dissipation of the young prodigal had brought him.

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