Don't get annoyed, it was just "zwanzer"!
The Brussels word "zwanze" refers to a hoax, often a rather heavy one, which takes advantage of the victim's gullibility to get a laugh. It involves the "zwanzeur" and his or her fall guy
Translating it as a joke would be a euphemism because a "zwanze" is much more than that. It requires imagination, banter, ridicule, daring, it involves a delicious mixture of Rabelaisian and Courtelinesque ingredients. The expression "to pull somebody's leg" comes near to it.
Here is an example of "zwanze", in the style of the biter being bitten, taken from a book on Brussels folklore, in which Louis Quiévreux refers to the case of a merry partner in crime known for his many "zwanze" who dug a well in his property. One morning, it started to give off a strong smell of oil. The man spent a thousand francs digging the well deeper, then noticed that he had been "zwanzed" by "zwanzers" (who had emptied a bucket of fuel into it) even better than him!
For André Goosse, the Belgian grammarian, "zwanze" comes from Dutch "zwans" meaning "pecker, male member", therefore, the term appears to have followed an evolution similar to the term "arse".