Past tense and past participle of dig (archaic or poetic form); the more common modern form is 'dug'.
Old English 'dician' or from Old Norse 'digga'. The '-ed' ending is the regular past tense suffix, but 'dig' became an irregular verb, with 'dug' becoming the standard form by the 16th century.
English is always regularizing its verbs—'digged' used to be standard, but 'dug' took over because it's shorter and sounds better. Some old English speakers resisted, which is why 'digged' appears in older texts and poetry.
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