A bet made wherein both involved parties realize that the other side does not intend to carry out their end of the bet due to the (most likely) ludicrous circumstances outlined in terms of the wager.
Sometimes though, for making such a bet, the loser has to do something small for the winner... just for making a stupid bet.
'Dude, look at that girl'
'Damn she's hot. Too hot for you.'
'Whatever, I bet you I can get in her pants by the end of the month'
a month later
'where's my $20?'
'Dude, it was an idle bet, you know that!'
'Yeah, you're right. Kiss my ring.'
'Kiss my ring. Or the $20, either one'