Is it correct that the phrase to put the boot in is not used in the US? If so, is there an equivalent?Yes, it's correct that that is a BrE phrase. The Collins Cobuild Dictionary defines it as:
If someone puts the boot in, they attack another person by saying something cruel, often when the person is already feeling weak or upset.Frank helpfully supplied some examples:
Mr Brown deployed a number of rehearsed lines against his two "rivals", the suggestion being that it will do him no harm to crush the left. But up against Mr Meacher - surprisingly hapless - and Mr McDonnell, there seemed little point, and each time he put the boot in I wanted to shout 'please don't hurt them'. The audience was overwhelmingly with the Chancellor. [Benedict Brogan, Daily Mail, 13 May 2007]In answer to Frank's second question, I can't think of an equivalent that is just AmE, but Frank describes it rather well as:
But European Commission President Jose Manuel Barroso has rejected the proposal in the strongest possible terms. Just seconds after the foreign secretary sat down he put the boot in, calling the proposal "unacceptable". [Mark Mardell, BBC News, 05 December 2005]
metaphorically kicking one's adversary while they are on the groundSo, to kick [someone] when [they're] down is a close alternative, though not exclusively AmE or BrE. Another near-equivalent is to twist (or turn) the knife, which again is not exclusively AmE or BrE.
So, as far as I can tell, BrE has at least one more idiom than AmE does for attacking someone in a weakened state. No comment. Unless you can think of a strictly AmE idiom for this sentiment?