A euphemism is a word or phrase used to avoid saying something unpleasant or offensive. We use euphemisms to say unpleasant things in a more polite way.
eg. economical with the truth = lie
a senior citizen = an old person
behind the times = old-fashioned
seen better days = to be old and in bad condition
getting on a bit = getting old
hard of hearing = a bit deaf
under the weather = to feel ill
a bit of a handful = difficult to look after
challenging = very difficult
a bit on the chilly side = cold