A dodgy situation is risky and suspicious. A dodgy person is skilled in lying and deceiving. Neither should be trusted.
When a person or situation is called dodgy, it's a clue that they should be dodged if at all possible. A job might be dodgy because it's illegal or dangerous. A shortcut might be dodgy if it could damage your car. Running a marathon without preparation is very dodgy. Similarly, a dodgy person is untrustworthy because they’re evasive, cunning, sly, or just a flat-out liar.