An archaic or obsolete past tense form of 'hallow,' meaning made holy or sanctified.
From Middle English 'halowe,' derived from Old English 'hālgian' (to make holy), with the obsolete -d past tense ending that competed with -ed in early English.
The '-d' ending (like 'hallowd') was common in Middle English before '-ed' standardized—it's the ghost of a completely different way English used to conjugate verbs that we abandoned.
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