From Old Galician-Portuguese erger (13th c., Cantigas de Santa Maria), from Vulgar Latin *ergere, from Latin ērigere. The modern verb has been rebuilt around the conjugation ergo (“I raise”), hence the other inflexions now all have /ɡ/ rather than the /ʃ/ one would expect from medieval forms like erger. Portuguese erguer and Spanish erguir were rebuilt in the same manner. Doublet of erixir, which was borrowed from Latin.