What does the "새" in 노새 and 버새 mean?

Answer: I'm not sure.

노새 is the Korean word for mule, a cross between a male donkey and a female horse; and 버새 is the Korean word for hinny, a cross between a female donkey and a male horse. A hinny is generally smaller than a mule, probably because a female donkey is smaller than a female horse. Anyway, the syllable 새 is in both 노새 and 버새, so I am curious about the meaning of 새, which in this case I am guessing does not mean bird. I am also curious to know the meanings of 노 and 버.

If anyone knows, has read something, or has any ideas, please let me know.