In a true manner; truly (archaic usage, now obsolete).
From Old English 'a-' (prefix meaning 'in a state of') + 'true,' used in Middle English to mean 'truly' or 'indeed.' The word fell out of use as 'truly' became standardized.
Old English loved adding 'a-' to words to create adverbs—like how we might say 'in truth'—but once English settled on more predictable adverb patterns, quirky words like 'adrue' simply vanished from the language.
Complete word intelligence in one call. Free tier — 50 lookups/day.