it's when you're quoting from someone else, and they've made a mistake, and you reprint their mistake in order to give a faithful quote.
E.g. say an MP, Ms Joan Smith, wrote a letter to you that said 'I can't help you with your prooblem.'
Then you write about this to someone else, you would write 'Ms Joan Smith wrote 'I can't help you with your prooblem (sic)
It means 'thus' in Latin, like saying 'this is what the stupid whatsit said in the first place and I know it's wrong but this is what s/he said'
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