Whenn is a common joke word, but shouldn't be. Well... I'm exaggerating. It's not a common joke, nor would it ever be useful in common parlance. However, within certain fields, it is perfectly obvious in meaning, and arguably useful. Whenn is the counterpart to iff; just as iff means "if and only if", whenn means "when and only when".
Iff is used to indicate a 100% correlation -- if X then Y, and also if Y then X. Or to put it more simply, these things only happen together. In English, the "most official" way to state a conditional relationship is with if; while there is no common way to state a biconditional, the official way is with iff.
English, however, is messy, and we state conditionals in a few different ways:
- If: "If it rains, I'll take an umbrella."
- When: "When it rains, I take an umbrella."
- Because: "Because it was raining, I took an umbrella.
- Unless: "Unless it's sunny, I'll take an umbrella.
Unfortunately, strong biconditional markers on because and unless would be... odd. But when is easy to modify, and in fact perfectly fits the established form. "I carry an umbrella when it rains, and only when it rains" becomes "I carry an umbrella whenn it rains"; the meaning is perfectly clear, and there can be no possible confusion.