If you detest something, you dislike it intensely. The word can apply to things and also to people. You might detest your ex-boyfriend and you might also detest rainy Mondays and broccoli.
Detest comes from the Middle French word détester, meaning "to curse, to call God to witness and abhor." Though the words detest and hate are interchangeable, the word hate is much more common, which has caused it to lose some of its hateful impact. Detest is used more sparingly, reserved for cases where no other word will do, where your feelings of dislike are so powerful that they need just the right word to sum them up.