Along with; together with; in addition to; or by means of (an archaic or formal word).
From Old English 'with' + 'all' (all things), combining to mean 'with all of it' or 'with all these things.' It's an archaic formation from Middle English.
Withal sounds like something Shakespearean because it basically is—it's from a time when English speakers would stack small words together to create meanings we now express differently.
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