What does Dill's explanation of jem's state of dress almost land him in trouble?

Dill lies to Atticus and says they were playing strip poker.... Atticus detests card playing and considers it gambling.

I admired my brother. Matches were dangerous, but cards were fatal.

"Jem, Scout," said Atticus, "I don’t want to hear of poker in any form again. Go by Dill’s and get your pants, Jem. Settle it yourselves."