If somebody makes a joke, but no one really understands what he said.
You might say that was so random（俚语）
(你或许会说，那真的是无厘头，或者像我们常说的那真是个冷笑话）

Let’s take close to look at this keyword “random”..

random 

n. 随意,任意
a. 任意的,随便的,胡乱的

英译：Adjective 

1. lacking any definite plan or order or purpose; governed by or depending on chance; "a random choice"; "bombs fell at random"; "random movements"

2. taken haphazardly; "a random choice"

词形变化:

副词:randomly 名词:randomness

例句与用法:

1.The librarian took a book at random from the shelf.

图书管理员从书架上随便拿了一本书。

2.soiled dishes were piled at random.
脏蹀子胡乱堆放着.

同义词

aimless   无目的，没有目标的

haphazard  偶然，偶然事件

irregular   不规则的

unorganized  无组织的

反义词

deliberate  深思熟虑的，故意的。

好了我们来看下几个比较实用的短语吧：

At random  随便地

He choose a book at random 他随便选了本书。

A random act of kindness （is something people do to help stranger, there is no particular reason for it）不经意的，不要求回报善事；随缘的。

For instance： Carry someone‘s grocery home, or giving up your seat on the bus, those are random act of kindness.