What is the Korean word for "turtle": 거북 or 거북이?

 ANSWER: Both words mean "turtle."

My Korean-English dictionary says that the Korean word for "turtle" is 거북, not "거북이," but in the Korean proverb listed together with the definition of 거북 the word 거북이 is used instead of 거북.

"거북이 잔등의 털을 긁는다." 

"It is very hard to shave an egg."

The above translation of the Korean proverb is not a literal translation. Here is the literal translation:

"A turtle is scratching the hair on its back."

One of the problems with many Korean-English dictionaries is that they seem to be written for Koreans, not for foreigners. If my dictionary, for example, had been written for foreigners, then wouldn't it have also given the literal translation of the Korean proverb?

Anyway, why does my dictionary list 거북 as the Korean word for "turtle" and then use the word 거북이 for "turtle" in the proverb? Or is the 이 in 거북이 just the subject marker?

I have read that 거북 and 거북이 both mean "turtle" but that even though 거북 is more commonly found in dictionaries, 거북이 is more commonly used by Koreans.

I have not read this anywhere, but it seems the word 거북이 may come from the Korean adjective 거북하다, which means "to feel shy" or "awkward." Or maybe the adjective 거북하다 came from the Korean word for "turtle"?

Since turtles hide inside their shells when they are approached, they appear to be "shy," so 거북이 may literally mean "a shy animal" since the suffix 이 can mean "animal," "person," or "thing."

If true, then 거북하다 could translate as "turtlelike."