Used after the same response is said by two people simultaneously. Often, a game is played where the person who failed to say "jinx" first becomes "jinxed", whereby they cannot speak until someone says their name
You can call something or someone that is considered to be unlucky or to bring bad luck a jinx. He was beginning to think he was a jinx. someone or something that brings bad luck, or a period of bad luck that results from this (Perhaps from jynx type of bird used in doing magic (17-19 centuries), from iygx)