ag (n.)

mid-15c., "tillage, cultivation of large areas of land to provide food," from Late Latin agricultura "cultivation of the land," a contraction of agri cultura "cultivation of land," from agri, genitive of ager "a field" (from PIE root *agro- "field") + cultura "cultivation" (see culture (n.)). In Old English, the idea could be expressed by eorðtilþ.

"college student studying agriculture," by 1880, American English college slang, from ag, abbreviation of agriculture, + -ie.