It's also worth noting that limericks tend to begin with a line such as 'There was an old man from ...', and they always have a set format for rhyme and rhythm. They go like this:

Rhyme:

A A B B A

A typical rhythm (where 'dee' is a weak syllable and 'dah' is stressed):

Dee dah dee dee dah dee dee dah

Dee dah dee dee dah dee dee dah

Dee dah dee dee dah

Dee dee dah dee dee dah

Dee dah dee dee dah dee dee dah

We can tell from your previous posts that you have an interest in all sorts of rude things (from sexual orientation to intimate body parts), but in fact most limericks aren't dirty.  Most are just silly. Here's a typical one:

There was a young lady named Bright

Whose speed was much faster than light

In a relative way

And returned on the previous night.

Nobody's mentioned yet that Limerick is the name of a city in Ireland.

As to why the verse form is called that, I'm convinced that the answer is "Nobody knows."

There once was a man from Milwaukee,

Whose face was all haggard and chalky;

He wore out his keys

Typing French and Chinese,

And spent way too much time on italki.

Here is a clean limerick about dirty limericks:

A limerick packs laughs anatomical

Into space that is quite economical;

But the good ones I've seen,

So seldom are clean--

And the clean ones are so seldom comical.

Here is another one. The joke here is that Fred's "verses," presumably a limerick, are "dirty." "Cleric" is a general name for religious leader, and they are supposed to be dignified and above dirty jokes. The bishop and dean are high-ranking clerics and pretend not to understand, but the curate, a lower-ranking and less experienced cleric, can't hide his feelings.

Your verses, dear Fred, I surmise

Were not meant for clerical eyes.

The bishop and dean

Cannot think what they mean--

But the curate turns pink with surprise.

Thank you all for your help.

To Peachey,

If google was not blocked here, I would do as you said. I kind of hate my country a little bit for having internet limits.

To Su.Ki

You made this word very clear to me and I love the example you gave to me, it's funny. Very appreciated.

Besides, I partly agree with you on where my interest is, but there are probably some misunderstandings in it.

I love learning all English expressions, and watching American comedy shows is one of my favorite ways for learning. fun fact is, all the comedies I have seen are full of rude terms or sex related terms as you said. In this case, I think American people and Chinese people share the same common ground that is rude terms or sex related terms are easy to make people laugh. so it's kind of healthy to learn them for getting the punch line not applying into life.(if even I wanted, I don't have many people around me can understand here in China).

On top of that, the rude things as you said involve lots of slangs and metaphors which normal things don't. so let's say normal things are easy to find and understand their meanings only by looking up in dictionary. but for rude things it's hard. Therefore I need to post them here for help. I think that's the reason why many of my posts are kind of rude things related. Actually I have learned lots of English phrases, not only them.