Leech words are by definition:
A leech is something that drains time from your studying and will keep doing so until you do something about it. The most essential example would be a piece of vocabulary, let’s say a character, that just refuses to stick in your mind. Even though it has appeared numerous times in your spaced repetition program, you just can’t recall the correct answer, perhaps because it’s very difficult, perhaps for no readily apparent reason whatsoever.
This is from Dealing with tricky vocabulary: Killing leeches | Hacking Chinese. I will occassionally publish a list of words in any given language I study on the board of shame here. These are the words that refuse to stick to my head right now (Mandarin):
- 勤奋 - hardworking, industrious
- 慷慨 - vehement, generous
- 生锈 - rust