If someone is trying to make people underestimate him for some purpose, what would be a word for this kind of person or action?
One term is sandbagging.
From the current Wikipedia entry for sandbagging:
Deceiving someone by pretending to be weak.
Another term with a fairly specific use is hustle. It's most often used in the context of pool (billiards) players, who downplay their skill in order to attract opponents. Once the newcomer gets comfortable winning games with relatively small bets, the hustler raises the stakes and wins a large sum quickly.
d. to lure less skillful players into competing against oneself at (a gambling game)
Merriam-Webster mentions the noun form hustler (which is the exact term for the 'someone' in your question), but doesn't have a separate definition it. From Dictionary.com:
- Slang. a person who employs fraudulent or unscrupulous methods to obtain money; swindler.
- Informal. an expert gambler or game player who seeks out challengers, especially unsuspecting amateur ones, in order to win money from them: