MAMMON

Biblical noun (NT)

1.

An Aramaic word meaning riches (3 Nephi 13:24)

MAMMON, form the Aramaic mammon, "wealth, riches" (possibly from ma'mon, from ’amn, "to trust," and meaning, literally, "trust, deposit" (see E. Klein, CEDHL, p. 352; cf. also discussion of Biblical Hebrew root ’amn in Köhler-Baumgartner, s.v.

If this is not an adoption of the English then we can appeal to the Aramaic. Aramaic was already in currency before the Babylonian exile, thus explaining the appearance of this Aramaic word in the Book of Mormon.