Origin of the expression "to do someone in"

I've seen the expression "to do someone in" for the first time today and from what I found, it means "to kill someone." Where does it come from?

The trailing "in" implies something following and makes the expression sound incomplete.

I'm sure the origin can be argued, but this almost certainly came from the east end of London.

When you say 'do him' or 'he did him' it means to beat someone up. So 'do him in' goes further.

Other common cockney phrases are 'Kick his head in' or 'punch his face in' so it only makes sense.

'In' being invasive, meaning entering inside, which is generally how you kill someone.

Coming from London myself it seems perfectly normal to to use 'in'.

I looked up the meaning of this expression as it is part of the Eliza Doolittle conversation during the Ascot Race scene. The British 'aristocrats' don't understand the expression and ask for the meaning. Professor Higgins has to explain it.In the context of the hilarious sketch, it is clear that the expression is 100% Cockney.