(adjective)
not able to be affected, so good as to be difficult/impossible to match or rival, or so powerful as to be above control or criticism;
e.g. The team set an untouchable record that has remained unbeaten for years. She rose to become untouchable in ranks of the company.
(adjective)
belonging to the lowest social group according to the Hindu caste system, considered to be impure or generally disgusting to touch;
e.g. That part of the city is inhabited by untouchable families.
(noun)
a person who belongs to the lowest social group in Hindu caste system, which used to be segregated from the rest of society due to contact with them being considered defiling;
e.g. He didn't want to speak to the untouchable for too long.