Folk etymology says a painter drew something half-horse (马) half-tiger (虎), and when asked what it was, he said 马马虎虎. His carelessness cost his sons' lives. The story teaches attention to detail.
usage
马马虎虎 (mǎmǎhūhū) is the reduplicated form meaning 'so-so' or 'passable.' A: 最近怎么样? B: 马马虎虎。