Pull one’s leg means is an idiomatic expression for standing one’s way.  In a Western island called Britannia, however, it is employed to mean to deceive a person playfully.

Could anybody make out why pulling someone’s leg is a metaphor of joking upon him?

Flam

Here’s what my 2107 Curious Word Origins, Sayings and Expressions from White Elephant to Song Dance says about pull one’s leg:

To coax, wheedle, blarney; bamboozle, delude, pull the wool over one’s eyes; befool, make fun of one.  Our cousins over the seas, among whom the expression originated, use it in the latter senses; the first is an American addition, carrying the tomfoolery into downright cheating and chiseling.  The Scots were apparently the first with the idea, using "draw" rather than "pull," as in the following quotation from a rhyme written in 1967:

He precched an’ at last drew the auld body’s ldg,
Sae the kirk got the gatherins [the money] o’ our Aunty Meg.

Just why one’s leg was somthing to be either drawn or pulled for the success of a delusion is most uncertain.  Perhaps it had something to do with tripping a person; i.e., to catch him in an error, or to bring him into a state of confusion.

Brazilian dude

Reminds me of German jemanden unter den Arm nehmen (take someone under one’s arm).

I guess the expression might have originated from human nature… I can see this happening, for example: Person A tells Person B about some inside joke.  Person A and Person B go to meet Person C, and later the same subject comes up.  Person A, while positioned beside Person B, tells Person C about the latest news, while tugging at Person B’s pants leg to signify the inside joke… probably sitting at the table or something, where Person C cannot see the pant tugging that’s going on.

That took way too much time to write down.  I hope it makes sense.  Not that I think it’s true, but just suggesting that it could be a possibility.

The same for taking someone under your arm, BD.  You would do that, for example, if you wanted to share a secret and not let anyone else hear it… I dunno.

-Tim