I am reading an W. Trevor story - A bit of business.
Two petty criminals went into Mr Livingston's house to rob. They found him watching TV then they warned him to stay quiet and tied him up in to his chair using ties. Before leaving the house, they partially choked Mr Livingston as a warning not to say anything by placing a tie round his neck and pulled two ends tight for a while.
Below was what the criminal thought after left the house couple of hours. He basically regrets not kill him first.
'Another couple of pulls of the tie, Mangan said to himself, and who’d have been the wiser? You get to that age, you’d had your life anyway. As it was, the old geezer had probably conked it on his own, tied up like that. Most likely he was stiffening already.'
Please could someone tell me what is meaning of underlined words. I do not find 'Conk on' in any dictionary. Without knowing this meaning, I do not understand meaning of 'stiffening' either.
Thanks for your time.