So many expressions in English make use of rhyming because the resulting euphony i.e. they just sound pleasant. Harum-scarum is yet another example. The expression dates from the late 17th century and is used to describe something reckless or careless. It is thought to derive from ‘hare’ and ‘scare’ and epitomises the way a hare runs when being pursued by hunters. The hare uses quick, jerky tactics, often doubling back on its own tracks as it 'hares' off. Some etymologists suggest that the expression may originally have been 'hare’em, scare’em', which makes admirable sense but does not provide conclusive proof, because the first known written example was always in the form of harum-scarum.