"byth bythoedd" meaning in Welsh

{
  "head_templates": [
    {
      "args": {
        "1": "cy",
        "2": "adverb"
      },
      "expansion": "byth bythoedd",
      "name": "head"
    }
  ],
  "lang": "Welsh",
  "lang_code": "cy",
  "pos": "adv",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Welsh entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Welsh entries with topic categories using raw markup",
          "parents": [
            "Entries with topic categories using raw markup",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Welsh terms with non-redundant non-automated sortkeys",
          "parents": [
            "Terms with non-redundant non-automated sortkeys",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "topical",
          "langcode": "cy",
          "name": "Time",
          "orig": "cy:Time",
          "parents": [
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "examples": [
        {
          "english": "From age to age and for ever and ever.",
          "text": "O oes i oes ac am byth bythoedd.",
          "type": "example"
        }
      ],
      "glosses": [
        "for ever and ever"
      ],
      "id": "en-byth_bythoedd-cy-adv-qhfG2lWv",
      "links": [
        [
          "for ever and ever",
          "for ever and ever"
        ]
      ],
      "synonyms": [
        {
          "word": "yn oes oesoedd"
        }
      ]
    }
  ],
  "word": "byth bythoedd"
}

{
  "head_templates": [
    {
      "args": {
        "1": "cy",
        "2": "adverb"
      },
      "expansion": "byth bythoedd",
      "name": "head"
    }
  ],
  "lang": "Welsh",
  "lang_code": "cy",
  "pos": "adv",
  "senses": [
    {
      "categories": [
        "Welsh adverbs",
        "Welsh entries with incorrect language header",
        "Welsh entries with topic categories using raw markup",
        "Welsh lemmas",
        "Welsh multiword terms",
        "Welsh terms with non-redundant non-automated sortkeys",
        "Welsh terms with usage examples",
        "cy:Time"
      ],
      "examples": [
        {
          "english": "From age to age and for ever and ever.",
          "text": "O oes i oes ac am byth bythoedd.",
          "type": "example"
        }
      ],
      "glosses": [
        "for ever and ever"
      ],
      "links": [
        [
          "for ever and ever",
          "for ever and ever"
        ]
      ]
    }
  ],
  "synonyms": [
    {
      "word": "yn oes oesoedd"
    }
  ],
  "word": "byth bythoedd"
}