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English Translation

lit.: a frog at the bottom of a well
fig.: a person with a very limited outlook, narrow-minded

Breaking it down

1. 井: well
2. 底: bottom, base
3. 之: possessive particle, literary equivalent of 的
4. 蛙: frog

Source: Google Image Search

Story in Chinese

一口废井里住着一只青蛙。有一天，青蛙在井边碰上了一只从海里来的大龟。 青蛙就对海龟夸口说： “你看，我住在这里多快乐！有时高兴了，就在井栏边跳跃一阵；疲倦了，就回到井 里，睡在砖洞边一回。或者只留出头和嘴巴，安安静静地把全身泡在水里：或者在软绵绵的 泥浆里散一回步，也很舒适。 看看那些虾和蝌虾，谁也此不上我。而且，我是这个井里的主 人，在这井里极自由自在，你为什么不常到井里来游赏呢！” 那海龟听了青蛙的话，倒真想进去看看。
但它的左脚还没有整个伸进去，右脚就已经绊 住了。它连忙后退了两步，把大海的情形告诉青蛙说： “你看过海吗？海的广大，哪止千里；海的深度，哪只千来丈。古时候，十年有九年大 水，海里的水，并不涨了多少；后来，八年里有七年大早，海里的水，也不见得浅了多少。 可见大海是不受旱涝影响的。住在那样的大海里，才是真的快乐呢！”
井蛙听了海龟的一番话，吃惊地呆在那里，再没有话可说了。

Source: http://chengyu.t086.com/gushi/79.html

Story in English

A frog once lived in a well. One day, the frog met a big sea turtle who came by the well. The frog boasted to the sea turtle: "Look, how happy I am to live here! Sometimes when I am happy, I jump around the railing for a while, when I get tired, I just return to the well and sleep next to the brick hole. Or just sticking out my head and mouth and soaking quietly my whole body in the water. Or, I like to talk a walk in teh soft mud, it is very comfortable. Looking at those small shrimps, who aren't any better than me. What is more, I am the master of this well, I can be free here, why don't you come here more often to enjoy!"
When the sea turtle heard this, it really wanted to go in to have a look. But before its left foot had entered, its right foot already hindered it. It quickly retreated two steps, and told the frog in the well about the big sea:
"Have you seen the sea yet? the vastness of the sea is thousand Li, the depth of the sea is a thousand zhang. In ancient times, out of ten years there were nine years of flooding, and yet the water in the sea did not increase by much. Later, out of eight years there were seven years of droughts, but the water in the sea did not decrease by much. Thus, the sea is not much affected by floods or droughts. To live in that sea, that is real happiness!"
When the frog in the well heard the words of the sea turtle, it was so surprised that it said nothing more.

Translation by Celine Li

Usage

你觉得这个就已经很好了的话那就只能证明你真是一个井底之蛙。
城市原来那么大，大得足以使我成为井底之蛙.