Erstick' is a shortened, colloquial form of ersticken, which means "to suffocate" or "to choke". It's a strong and dramatic verb that immediately conveys intense emotion.
In the song, the line "Weil ich an deinem Nichts erstick'" (Because I suffocate on your nothingness) uses this powerful word to express a feeling of being overwhelmed and consumed by the emptiness or absence of the other person. It's a vivid metaphor for emotional despair, making it a memorable and impactful word choice.
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