The Collaborative International Dictionary of English v.0.48:

Therewithal \There`with*al"\, adv.
   1. Over and above; besides; moreover. [Obs.] --Daniel.
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            And therewithal it was full poor and bad. --Chaucer.
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   2. With that or this; therewith; at the same time.
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            Thy slanders I forgive; and therewithal
            Remit thy other forfeits.             --Shak.
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            And therewithal one came and seized on her,
            And Enid started waking.              --Tennyson.
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WordNet (r) 3.0 (2006):

therewithal
    adv 1: together with all that; besides; "thy slanders I forgive;
           and therewithal remit thy other forfeits"- Shakespeare