In a state of bloom or flowering; blooming or blossoming.
From 'a-' (in, on) + 'bloom.' This construction follows Old English patterns where 'a-' was used to indicate a state or condition, similar to 'aflame' or 'afire.' The term became standard in poetic English by the 19th century.
Poets loved 'abloom' for gardens and romance scenes because it sounds more lyrical than just 'blooming.' But we've nearly lost this whole family of 'a-' state words—try saying 'the garden is abeautiful' and you'll feel how weird it sounds compared to 'abloom'!
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