The root of relent is the Latin "lentus," which means "to slow down or soften," and the original meaning (from the 15th century, no less) most likely had to do with the heart — as in "to stop resisting love". Now, however, relent can have all manner of relevant applications, but the meaning is always the same: to let up, soften, yield or give in. Often, you'll hear the adjective relentless used to describe someone who won't relent: "The firemen refused to relent; they banged relentlessly on the door until I woke up."