Fallon (FAHL-on), noun: 1. A comedian unable to keep composure during even the lamest “Saturday Night Live” sketches; 2. An “SNL” cast member who lacked even one good recurring character; 3. The, um, “actor” who co-starred in 2004’s wretched “Taxi”; 4. Likely to join Chevy Chase and Magic Johnson as late-night talk-show hosts consigned to history’s scrap heap.