In Genesis 24:2 it reads, "And Abraham said unto his eldest servant of his house, that ruled over all that he had, Put, I pray thee, thy hand under my thigh:"
Why did he have to put his hand under his thigh? Was is the meaning of this?
We know very little about the significance of this style of taking a vow. The only other place it is mentioned in in Genesis 47:29. It doesn't appear in any historical account outside the Bible, so we have nothing with which to compare it.
The typical guess is that the vow represents being made before the person and his future generations. Another possibility is that since people in ancient days carried a sword at their side, against their thigh (Psalm 45:3), that it might be a symbolic representation of accepting the sword as a consequence for failing to keep the vow. It should also be noted that the one with his hand under the thigh was in an awkward, subservient position, unable to defend himself, so the vow done in this way may indicate a willingness to bend before another person's will.