In progress; happening or continuing; moving forward (archaic or dialectal term).
From a- (on) + going, a phrasal combination meaning 'in a state of going' or 'actively in motion,' common in older English.
This word is basically the ancestor of modern phrases like 'things are really moving' or 'we've got this thing going'—it shows how English has always reused the verb 'go' to mean 'make progress.'
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