What is the meaning of 〜てやんな?
For the sentence
「許してやんなよ」
the English translation is:
'Just give in.'

I thought 「やんな」 would be slang for 「やるな」 meaning to 'not do [something]'? But this isn't the case?

Question

What is the meaning of 〜てやんな?
For the sentence 
「許してやんなよ」
the English translation is:
'Just give in.'

I thought 「やんな」 would be slang for 「やるな」 meaning to 'not do [something]'? But this isn't the case?

Answer

Basic sentence is 「許してやってください」or 「許してやって」
It means " Please forgive him (her)." " Just forgive him (her)" This is when you are fighting with your friend, and third party said to you「許してやんなよ」because fighting is over, he already apologize to you.
It's hard to explain this... hope you understood it.

Answer

It means 許してやりなよ.