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From The Collaborative International Dictionary of English v.0.48:
Guano \Gua"no\, n.; pl. Guanos. [Sp. guano, fr. Peruv. huanu dung.] A substance found in great abundance on some coasts or islands frequented by sea fowls, and composed chiefly of their excrement. It is rich in phosphates and ammonia, and is used as a powerful fertilizer. [1913 Webster]