What does the expression take it on head mean?
I've not heard this expression. Take it on the chin means to take a verbal beating (alluding to a boxing match).

He means that he 'won (ie took) the prize while standing on his head'. This is a figurative way of saying that he won it very easily.

Do you know what 'stand on your head' literally means? Your hands are supporting you on the floor, and your feet are high above your head, like a gymnast. We also call this 'standing on your hands'.

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Can you please mention the context sir? Then i'll try answering this.

Please do not address me sir. We will be more than glad if you could use 'please' and 'thanks' wherever necessary.

Here is the context. It is from "Keeping it from Harold" by P G Wodehouse

"It jarred upon him, this habit of his mother's of referring to herself in the third person, as if she were addressing a baby, instead of a young man of ten who had taken the spelling and dictation prize last term on his head."
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 Clive's reply was promoted to an answer.
Thank you, Clive.