WordWeb Online

1. Painstakingly careful and accurate
   "thorough research"; "our accountant is thorough";
   - rigorous
    
2. Performed comprehensively and completely; not superficial or partial
   "made a thorough search";
   - exhaustive, thoroughgoing, rigorous, in-depth
    
3. Without qualification; used informally as an (often pejorative) intensifier
   "a thorough nuisance";
   - arrant, complete, consummate, double-dyed, everlasting, gross, perfect, pure, sodding, stark, staring, thoroughgoing, utter, unadulterated, rigorous

See also: careful, complete, unmitigated

Encyclopedia: Thorough