Well the 'Big Knife' reoccurs several times, in many different places. Its first appearance is in the 'A guy with a big knife' painting, from the StrongBadEmail/i love you. It also returns in StrongBadEmail/army.
It is also being held by the snake on the Strongbadia flag.

Strong Bad also refers to the Big Knife in StrongBadEmail/band names, where one of the band names he suggests is "Bigg Nife".

Strong Bad also asks the cheat if he could borrow that Big Knife via instant messaging in StrongBadEmail/marzipan

The art test you must complete to get into Crazy Go Nuts University has a picture of a "guy w/ big knife" that you have to draw.

Also, Stinko Man can be seen holding the knife in StrongBadEmail/japanese cartoon.

He also throws it at Strong Sad in the Theme Song Video.

Early on in Everybody to the Limit Strong Bad is holding it again.

Also, Strong Bad uses the knife to cut a hole in the couch to hide his Aztec gold in StrongBadEmail/couch patch

In the toon Wheres The Cheat, there appear to be big knives in Marzipan's kitchen