The Collaborative International Dictionary of English v.0.48:

Annotto \An*not"to\ ([a^]n*n[o^]t"t[-o]), Arnotto \Ar*not"to\
   ([aum]r*n[o^]t"t[-o]), n. [Perh. the native name.]
   A red or yellowish-red dyeing material, prepared from the
   pulp surrounding the seeds of a tree (Bixa orellana)
   belonging to the tropical regions of America. It is used for
   coloring cheese, butter, etc. [Written also Anatto,
   Anatta, Annatto, Annotta, etc.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Arnotto \Ar*not"to\, n.
   Same as Annotto.
   [1913 Webster] Aroid