In a state of dryness; dried or thirsty (archaic).
From Old English 'a-' (prefix indicating state) + 'dry.' This construction was common in Old and Middle English but became obsolete as English shifted to using 'dried' or other forms.
Notice how 'adry' works like 'ablaze' or 'afloat'—all these 'a-' words describe a state or condition—but we've mostly stopped making new ones this way, which is why 'adry' sounds so strange to modern ears.
Complete word intelligence in one call. Free tier — 50 lookups/day.