The word translated here "foolish" means unintelligent or unwise, and by implication sensual. This implication is very interesting when considered in light of what the letter to the Galatians is fundamentally about: The Galatians were trying to use the rites and ceremonies and physical requirements of Gnostic Judaism to "work" their way into God's Kingdom. Their emphasis was on what they were doing, rather than on God's work in them. Their focus was on things dealing with the senses; things that would be, by definition, sensual—not in terms of being sexual or provocative, but rather indicating the emphasis on the physical senses.
This word (anoeetoi—Strong's #453) is a derivative of a negative particle and noeo (Strong's #3539), which means to exercise the mind, observe, to comprehend, heed, consider, perceive, think, or understand. So the word foolish is the opposite (because of the negative an) of all these things. The Galatians, then, were not exercising their minds; they were unobservant, uncomprehending, unheeding, inconsiderate, imperceptive, non-thinking, and non-understanding. They were not thinking things all the way through, and not fully considering all of the aspects of the way they were living. They were unable to see that their ideas and views did not add up—that there were some obvious gaps in their understanding that had brought them to the condition they were in.
Paul here is continuing with a theme from Galatians 1:4-9—namely, that the Galatians were falling away ("so soon removed") from the original teaching that had been given to them by God through His human servants. The very foundation of the New Covenant with God is that we can build a relationship with God directly—because of the sacrifice of Christ. For them even to make the covenant with God properly, it was a requirement that they understand that justification by means of a sinless sacrifice was the only way it is possible for us to come into God's presence! Our own righteousness is as "filthy rags" in comparison to God's; our works simply do not amount to enough to even out the scales. But this does not negate the necessity of working! The Galatians' problem was that they thought their personal righteousness was sufficient—and if that was the case, then truly there was no need for Christ to die.
Paul refers to the Galatians being "bewitched." This word means "to malign," or "to fascinate by false representation." The Galatians were drawn in—their fascination was piqued by these Jewish and Gnostic ideas. It did not take long for them to begin slipping spiritually, and a large part of this was because of their misplaced faith. They had more faith in themselves, in their own works, to save them than they had in Christ's crucifixion, resurrection, and intercession! They did not see or know God clearly enough, and the absence of Him in their lives created a void that was quickly and easily filled by these false ideas.
This is the only place in the New Testament where this word ("bewitched") is used (Strong's #940), but numerous other verses speak of this principle. Paul is speaking of this principle when he says in Galatians 1:7-9 not to deviate from this gospel message even if an "angel" from heaven gave them different instructions! The Galatians were weak enough in the faith that they could be easily deceived and drawn away if one of Satan's angels were to appear before them.
Matthew 24:24 speaks of false Christs—false ideas, pictures, impressions about Christ—arising, as well as false prophets, who will be able to manifest terrific signs and wonders to the extent that even the elect of God could be deceived. This is why we have to have such a concrete picture in our minds of what "Christ" is comprised of so that when we begin to hear about or see miraculous things, our faith will not be shaken as the Galatians' was.