Probably misinterpretation (in Dict. Amer. Engl., followed by O.E.D. Suppl. and other dictionaries) of ‘old field-preacher’ (see field preachern.), taking ‘old field’ as a noun compound used attributively (see old fieldn.).
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1904 T. WatsonBethanyii. iii. 168 The tremendous emphasis with which the old field preacher uttered the words.
This entry has been updated (OED Third Edition, March 2004; most recently modified version published online March 2018).