1610s as an action; 1756 as a substance; verbal noun from frost (v.). Specific meaning "cake icing" is by 1832, so called from its appearance.
Entries linking to frosting
frost v.
1630s, "to cover with frost," from frost (n.). Intransitive sense of "to freeze" is from 1807. Related: Frosted; frosting.
icing n.
1769 in the confectionery sense, "coating of concreted sugar," verbal noun of ice (v.). Earlier in this sense was simple ice (1723); frosting came later. Meaning "process of becoming covered with ice" is from 1881.