And in Scrabble it's fine to use a nonsense word if nobody challenges it.

And in American Football it's fine to deflate the ball if no one measures it.

Not the same. Because there is a penalty to challenging a word that turns out to be a real obscure word found in the dictionary (the official Scrabble dictionary if you playing seriously), it is a legitimate strategy (not cheating) to use obscure words early on, be proven correct when challenged, and then use fake words that opponents are wary of challenging.

This is not implicit though.

The rules state that property owners can ask for the rent until the next player rolls the dice.

In Bullshit, I always felt that if you could trick them into taking the hand or not noticing you added an extra card, it was all part of the game.

3 queens ... and slide a 4th card sideways underneath.

Yep.

IMO that's still not cheating