It's a quotation from Horace's Satires 1.4.34:

'faenum habet in cornu, longe fuge;

"He has hay on his horn, keep away; ..."

See footnote 6 on this page: http://perseus.uchicago.edu/cgi-bin/ptext?lookup=Hor.+S.+1.4


And Plutarch's discussion of the custom in his Roman Questions: http://makeashorterlink.com/?B29A111C6


Bingley

Bingley