When you remark that a specific transaction was above board, you're saying that there was no hanky panky involvement. In other words, it was both honest and lawful. The type of arrangement you don't often associate with our politicians. If a politician is engaged in a transaction, you can bet it's not legal.

Raju is straight as they come. All his dealings are above board.

There are various theories as to where this idiom came from. According to one, the 'board' refers to a table. When you say a deal was 'above board,' you suggest it was done in an open way — the hands of the parties in the trade were out in the open. There was no fraud involved. Another explanation is the smuggling of items on ships. Smugglers used to keep their stuff hidden below deck. The legal load would be displayed on the deck. In other words, everything placed on the deck was declared 'above board'!