Definition of "thoro"

… the Roller Coaster is to get a thoro overhauling.

1946 April 6, “Thoro Overhauling for Manlius Rides”, in Billboard‎, page 81

Meanwhile, he said he has ordered a thoro government audit of all major oil company records

1974 January 20, “No Gasoline Rationing Seen”, in Chicago Tribune, Chicago