According to the Bible, the dowry that David gave to Saul for his bride, Michal.

1 Samuel 18:25-27:

When David wanted to marry the king Saul's daughter, he asked David to bring to him a hundred foreskins of the Philistines, that he may be avenged of the king's enemies.

David arose and went, along with his men, and killed two hundred of the Philistines; and David brought their foreskins which were given in full number to the king that he might become the king's son-in-law.

Moral: If you have a rough relationship with your father-in-law, follow the Biblical example: go out and kill 200 guys he doesn't like, then surprise him with a gift of 200 foreskins.

If you read 1 Samuel you'll discover that Saul is continually jealous and suspicious of David. I always interpreted this passage as an example of an "impossible task" motif of folklore: the father who doesn't like a particular young man gives him an apparently impossible task--usually one that is expected to result in his death. Of course, the young man goes out and achieves this impossible task--and in this case, David goes the extra mile and kills another hundred Philistines, just to make his point clear.