Underhanded means done secretly and dishonestly. Underhanded tactics, an underhanded deal, thoroughly underhanded.
The shorter form underhand means the same thing in Britain and is commoner there. The image is of a hand kept below the table, which is exactly what the accusation says.
1adjectiveunderhandedDone secretly and dishonestly.
“Spreading false stories about a rival is an underhanded way to win votes.”
underhanded is a quality.
From under- + handed.
Etymology from Wiktionary, CC BY-SA