Definition:
: something highly and usually factitiously sensational of its kind
About the Word:
Compilers of dictionaries generally follow a set of rules when crafting their definitions.
One of these is that the definition should have no words or terms in it that are more difficult to understand than the word that is being defined. Every once in a while one will slip through, as may be seen in the above definition for killer-diller from the Merriam-Webster Unabridged dictionary. However, in this case, the phrase in question ("factitiously sensational") has such a pleasing ring to it that it may be excused.
Killer-diller comes from duplication of the slang term killer, which, in addition to referring to a person who kills things, has served to denote something that is spectacular.
Example:
"It seems Ralph is the steady type and Randy is the killer-diller with the ladies. Both are fliers — and buddies." — S. R. Mook, Silver Screen, August 1939
