Dob, Dob In, Dobbin

It turns out that Dobbin is an old word. It appears, for example, in Act II of Shakespeare's The Merchant of Venice, when Shylock's attendant meets with Gobbo and discovers he is his long-lost son.
Gobbo says:

... if thou be Launcelot, thou art mine own flesh and blood.
Lord worshipped might he be! what a beard hast thou
got! thou hast got more hair on thy chin than
Dobbin my fill-horse has on his tail.

Dobbin is a diminutive form of Dob, which is short for Robin or Robert.

But then 'dob' has another meaning. To dob means to put something down heavily or to throw something down heavily and to 'dob in' means to contribute towards the cost of something, for example a leaving present for a co-worker.

You can imagine someone tossing their contribution into the pot in that nice off-hand way that people do when they want to preserve their modesty and not seek to attach too much importance to their contribution.

But in Australian English, to 'dob in' also means to give someone up to the authorities. I thought it was a colloquialism or something only said in casual speech, but the Department of Immigration and Citizenship of the Australian Government has a web page advertising its toll-free Immigration 'Dob-in Line' which you can call to advise the department about a person living in Australia illegally.

So the word 'dob' has woven a trail from a friendly horse in a field in the north of England around the world to the Australian Government's efforts to catch illegal aliens. How strange