1. Secret; clandestine, usually implying meanness or fraud, or both.

2. Underhand.

3. By secret means; by fraud.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• undertand,
• undetand,
• undersnd,
• underhood,
• underwant,
• undirtand,
• underpant,
• undersand,
• andertand,
• undersend,
• undersant,
• underand,
• underpend,
• underhaned,
• underhande,
• undrhand,
• underhnd,
• undrehand,
• underhhand,
• underhaand

• underbrush,
• unadorned,
• undersexed,
• underworld,
• underhung,
• underwater,
• underplay,
• undergo,
• underpin,
• understaffed,
• underbodice,
• under wraps,
• undersized,
• underdog,
• underling,
• understudy,
• underclass,
• underline,
• undergird,
• undergrad,
• under that,
• underfur,
• undercoat,
• underwear,
• untier,
• undersea,
• under arms,
• underhill,
• undercharge,
• Underdahl,
• undersealed,
• undersign,
• underbody,
• understood,
• underwood,
• understated,
• undermine,
• underwing,
• undertide,
• underneath,
• underseal,
• undercoated,
• underfed,
• underspend,
• under way,
• underbid,
• understand,
• underdown,
• underdone,
• undoer,
• underhanded,
• underpants,
• underdress,
• undermanned,
• underpart,
• underbred,
• undertone,
• underived,
• underground,
• undertaker,
• understate,
• underframe,
• undersurface,
• undershrub,
• underside,
• undercover.

The adjective form of the word "underhand" is "underhanded".