/meuh koy"/ , n.
the genuine thing or person as promised, stated, or implied (usually prec. by the or the real ): Those other paintings are copies, but this one is the McCoy.
[ 1880-85; also Mackay, McKie, the clear McCoy (of liquor); of uncert. orig.; hypothesized identifications with Mackay, a Scottish clan, and Kid McCoy, nickname of U.S. boxer Norman Selby (1873-1940), are unsubstantiated ]