Albert Barnes Commentary Deuteronomy 29:18

"lest there should be among you man, or woman, or family, or tribe, whose heart turneth away this day from Jehovah our God, to go to serve the gods of those nations; lest there should be among you a root that beareth gall and wormwood;" — Deuteronomy 29:18 (ASV)

The word rendered “gall” here and in Deuteronomy 32:32, is translated “hemlock” in Hosea 10:4. It is the name of a plant of intense bitterness and quick growth, and is therefore repeatedly used in conjunction with “wormwood” (Lamentations 3:19; Amos 6:12), to express figuratively the nature and effects of sin (compare the marginal references).

The herb is probably the poppy. Therefore, the “water” (that is, juice) “of gall” (Jeremiah 8:14; Jeremiah 23:15) would be opium. This would explain its use in the stupefying drink given to criminals at the time of execution (Matthew 27:34) and the use of the word as synonymous with poison (Job 20:16).

Wormwood - is the plant “absinthium.” It is used to denote metaphorically the distress and trouble that result from sin.

“The root that beareth gall and wormwood,” means in this place any person lurking among them who is tainted with apostasy.