Old English 'all so' — and the tiny word 'as' is its worn-down twin, both born from the same compound.
In addition; too; besides; as well.
From Old English 'eallswā' (entirely so, just as, also), a compound of 'eall' (all, entirely) + 'swā' (so, as). The word literally meant 'wholly so' or 'just as' — a strengthened form of 'so.' Over time, the meaning shifted from 'in exactly that manner' to 'in addition,' as the idea of complete likeness ('entirely so') became associated with adding another similar item. The parallel formation 'as' is a weakened descendant of the same compound. Key roots: eall (Old English: "all, entire, wholly"), swā (Old English: "so, as, in that manner").