I'll swing for you
"I will kill you and am prepared to be hanged as a consequence", or "I will swing a punch at you".
Queries about this phrase have often been posted on the site's discussion board and I have always been surprised at the responses. Most respondents appear never to have heard the phrase used to mean 'I will take a punch at you', whereas I had spent a lifetime (n the UK) having never heard it being used to mean anything else. It seems that, as well as being in the minority, in thinking there was only one meaning to this phrase I have been under a misapprehension all these years. It is quite reasonable, linguistically at least, to say 'I'll swing for you' when threatening to punch someone, but that isn't how the expression originated.
'Swing' has been used as a slang term meaning 'hang' since the 18th century and was so defined in The New Canting Dictionary, 1725. All of the early citations of 'swing for you', 'swing for her' etc. refer to the 'prepared to suffer hanging' meaning. The first printed example I've found is in a rather unlikely source - The Lady's Magazine, 1787. The reference comes in the dialogue of a comedy called The Embarrassed Husband:
"Murder him? No, no - it is not worth while to swing for him."
The magazine was a typical women's magazine of the Regency period, claiming to be 'The entertaining companion for the fair sex, appropriated solely for their use and amusement' - they don't write them like that any more.
The confusion in the meaning of the phrase is perhaps understandable. When announcing plans to murder someone, saying 'I'll swing for him' could easily be misinterpreted to mean 'I'll swing my arm at him'. Also, even when understanding the 'suffer hanging' interpretation there could be confusion between 'I dislike you so much I am prepared to be hanged so long as I get the pleasure of killing you first' and 'I like you so much I am prepared to kill and suffer hanging on your behalf'.