John S. Farmer, Slang and its analogues past and present…
“ Grose, Vulg. Tongue, s.v. Greenhorn (or Green-Head, or Greenlander) , subs. (common) .—A simpleton; a fool; a gull (q.v.) ; also a new hand. For synonyms, see Buffle and Cabbage-head. To come from Greenland = to be fresh to things; raw (q.v.) . Greenlander sometimes = an Irishman. 1753. Adventurer, No. 100. A slouch in my gait, a long lank head of hair and an unfashionable suit of drab-coloured cloth, would have denominated me a greenhorn, or in other words, a country put very green. 1815. Scott, Guy Mannering, ch. xliv. ”
