Toutacoo is a colloquial and slightly archaic spelling of "tout à coup," which means "all of a sudden" or "suddenly."
In the context of the song, it dramatically sets the scene for the grandfather's unexpected appearance, adding a touch of old-fashioned charm and immediate intrigue to the narrative. It's a less common form than the standard "tout à coup," making it stand out.
This song, "Tout à coup, Grand-Père apparut" by Daniel Lavoie, tells a short, dramatic story about a boy named Pierre and his grandfather. It starts with the sudden appearance of the grandfather, who is clearly upset because Pierre went into the meadow. The grandfather warns Pierre that the area is dangerous, especially if wolves were to come out of the forest, asking, "que ferais-tu" [what would you do]?
But Pierre, being a typical boy, ignores his grandfather's concerns, boldly declaring that "les grands garçons n'avaient pas peur des loups" [big boys aren't afraid of wolves]. This defiance doesn't sit well with the grandfather, who then takes Pierre by the hand, leads him home, and locks the garden gate. It's a classic tale of a child's adventurous spirit clashing with an elder's protective wisdom, all wrapped up in a simple, memorable narrative.