Origin
This saying is attributed both to Joseph P. Kennedy (1888-1969), father of the USA President John F. Kennedy, and to Norwegian-born American football player and coachKnute Rockne (1888-1931).
Explanation
When things are difficult, strong people take action and do not despair. (Can be used for encouraging someone to take action). It is said to emphasize that when conditions become difficult, strong people take action. It is mostly used to encourage someone to pull out in a difficult situation. It means that strong people do not lose their heart and face the challenge bravely in a hard situation. In context, “the going” means “the situation,” “gets tough” means “becomes difficult,” “the tough” means “people who are strong or enduring,” and “get going” means “become fully engaged.” Taken together, the meaning of the phrase is: When the situation becomes difficult, the strong will work harder to meet the challenge. When the going gets tough only those who are tough enough or have the necessary will-power, will triumph/succeed.
Examples
The football team was losing the game, so at half time the coach reminded them that when the going gets tough, the tough get going.
Jill: I don’t think I can walk all the way to the top of this hill; it’s so steep!
Jane: Don’t give up. When the going gets tough, the tough get going.