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1. A coat worn beneath another coat.
2. A covering of short hairs lying underneath the longer outer hairs of an animal's coat.
3. also un·der·coat·ing (-kō′tĭng)
a. A coat of sealing material applied to a surface before the outer coats, as of paint, are applied.
b. The sealing material used for this purpose.
c. A tarlike substance sprayed on the underside of a vehicle to prevent rusting.
tr.v. un·der·coat·ed, un·der·coat·ing, un·der·coats
To apply an undercoat to.
un•der•coat•ing(ˈʌn dərˌkoʊ tɪŋ)
a protective seal applied to the underside of an automobile.