Green’s Dictionary of Slang

(US) a portmanteau description of the East coast preparatory schools St Marks, St Paul’s, Groton and Middlesex.

1981		J.K. Galbraith A Life in Our Times 51: We were told to accept automatically all the St. Grotlesex and, generally speaking, all the E. & A.’s.
2004		at www.whoi.edu 🌐 As was appropriate for the scion of a New York banking family, Columbus attended one of the ‘Saint Grotlesex’ preparatory schools – St. Mark’s in Southboro, Massachusetts.