In a very unhappy, uncomfortable, or inadequate way; in a manner showing misery or poor quality.
From 'miserable' (Latin 'miserabilis': 'miserēri' to pity, from 'miser' wretched) plus adverbial suffix '-ly.' The sense evolved from 'deserving of pity' to 'causing discomfort' to the modern sense of extreme unhappiness.
The Latin root 'miser' literally meant 'deserving of pity,' but modern English also uses 'miser' for someone who hoards money—two completely opposite character types sharing the same root because both were seen as pitiful in medieval times!
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