He did it off his own back

You hear that a lot these days, so much so that it’s entered into common use. In fact, the original expression is ‘He did it off his own bat’. The bat in question is a cricket bat and the first activity that was said to be done ‘off someone’s own bat’ was to score runs. The idiom conveys the idea of someone doing something independently, without prompting.

Each one worse than the next

This phrase makes no sense if you pause to think about the meaning of the words. You can’t say something is worse than the next because you don’t know what the next one will be. But you do know what the last thing before it was. The idiom ‘each one worse than the last’ is more logical.

I could care less

This makes more sense if we say ‘I couldn’t care less.’ If you could care less, there is an opportunity for you to care less about the subject at hand. This flawed variant seems to have entered popular usage in the 1960s.

You’ve got another thing coming

The accepted phrase ‘you’ve got another think coming’ can be traced back to an original idiom, ‘If that’s what you think, you’ve got another think coming.’ This idiom is intentionally ungrammatical, so modern users find it more natural to land on ‘another thing.’

It’s a doggy dog world

This idiom suffers in the slurring of words, but the meaningful phrase ‘dog eat dog’ refers to when everyone is looking out for themselves with no concern for the harm done or other people’s feelings.

First come, first serve

This one just comes down to verb tense. The idiom means that the first one to arrive is served first and it’s in the past tense ‘first come, first served’. The full phrase could be written out as ‘the first to come will be the first to be served,’ and this version helps clarify the need for the past tense.