Tokko is either short for tokkoutai (特攻隊), which means ‘special attack unit’ or ‘suicide squad’, or tokkouyaku (特効薬), ‘wonder drug/silver bullet’, so shortening it down to where it could be either is probably a Japanese play on words referring to the show’s main focus: A specialised squad (with a short lifespan) who’re designed to be the ‘cure’ to supernatural threats. The show won’t tell you, because it’s a japanese TV show which probably assumes you understand the language!