Definify.com

Webster 1913 Edition

Noun.

[See

A mixtire of caustic lime and orpiment, or tersulphide of arsenic, – used in the depilation of hides.

Knight.

rhusma ‎(usually uncountable, plural rhusmas)

A mixture of caustic lime and orpiment, or tersulphide of arsenic, used in the depilation of hides.
(Can we find and add a quotation of Knight to this entry?)