Merriam-Webster Collegiate Dictionary - greenhorn

 noun  Etymology: obsolete ~ animal with green or young horns  Date: 1682  1. an inexperienced or naive person  2. a newcomer (as to a country) unacquainted with local manners and customs GREENHOUSE EFFECT  noun  Date: 1937 warming of the surface and lower atmosphere of a planet (as Earth or Venus) that is caused by conversion of solar radiation into heat in a process involving selective transmission of short wave solar radiation by the atmosphere, its absorption by the planet's surface, and reradiation as infrared which is absorbed and partly reradiated back to the surface by atmospheric gases