It means how would you like your spanking/punishment for betting in the first place? Over the top where you see it coming, or, under when you least expect it.
In betting, an over/under bet is one where the bookmaker makes a prediction about the numerical result of a contest, or multiple contests.
For example, if Disko Tech is playing a football game against Whatsamatta U., they might predict the combined score of both teams in the game to be 48 points (e.g. one team beats the other 27-21). Bettors would then bet whether the actual game combined score will be more than 48 points (the "over") or less than 48 points (the "under")
The goal of the bookmaker is to get as close as possible to having exactly as many people bet the "over" as bet the "under." If that happens, the losers pay off the winners, and the bookmaker skims a percentage for setting up the action. Hence, over time, the bookmaker will move the demarcation number to attempt to get more bettors on one side or the other. If many people better the "over" in the hypothetical DT vs. WU football game, the bookmaker might move the prediction from 48 points to 51 points in order to lure more people to bet the "under."
Such bets can be devised for almost any numerical quantity, such as how many games a team will win in a season, or how many home runs a baseball player will hit in an upcoming year, and so forth.
The language of over/under is sometimes used (by people familiar with this terminology) to indicate a rough prediction of a likely outcome for some uncertain event. For example...
Person #1: "Joe and Jane just got married. How long do you think they'll stay together?"
Merlin 