Is there a term for "purposely deceptive behavior"?

Solution 1:

The term I usually hear is

sandbag v, v tr
To downplay or misrepresent one's ability in a game or activity in order to deceive (someone), especially in gambling: sandbagged the pool player by playing poorly in the first game when stakes were low.
^_TFDO

The etymology is interesting and instructive. From Etymonline:

. . . Meaning "pretend weakness," 1970s perhaps is extended from poker-playing sense of "refrain from raising at the first opportunity in hopes of raising more steeply later" (1940), which perhaps is from sandbagger in the sense of "bully or ruffian who uses a sandbag as a weapon to knock his intended victim unconscious" (1882).

I take from this that the sandbag so mentioned must have been a non-obvious weapon that would have taken the victim by surprise.

Solution 2:

In the context of gambling, you can use hustle:

To lure less skillful players into competing against oneself at (a gambling game)

(Merriam-Webster)

For other contexts, there's the idiom play dumb:

To pretend to be slow-witted or lacking in specific knowledge, usually in order to avoid responsibility or to gain some advantage

(Wiktionary)

TV Tropes also has a whole page on this phenomenon, which it calls Obfuscating Stupidity.