Where did the phrase holy guacamole come from?

The noun 'guacamole' is a Spanish-American Word originating from the Nahuatl, or Aztec, term ahuacamolli: ahuacatl(avocado) plus molli (sauce, paste).

It still has the same meaning today: a sauce or paste made from mashed or pureed avocado, usually crushed with lemon or lime juice and aromatics such as onion, garlic and other seasonings, sometimes with tomato, and served as a dip or spread, or an accompaniment to other foods.