Not that much is known about the Roman poet Juvenal, who lived in the late-1st/ early-2nd century AD, but he coined a number of phrases which are still common today, and some of which are still said often in Latin. These expressions come from his Satires, five books of poems.
bread and circuses: Juvenal used the term contemptuously to refer to the concerns of his fellow Romans, who had no interest in heroism or any higher qualities, but only in trivialities and diversions (as provided by the government as a way of appeasing the masses).
a sound mind in a sound body, often said in Latin, mens sana in corpore sano: It is one of a list of things that the poet considers people should desire in life.
rare bird, often said in Latin, rara avis: the translation of Juvenal's line is "a bird rare upon the earth and very like a black swan". 'Black swan' is in the OED, as is rara avis, and, in fact, is defined as a 'rara avis'.
who will watch the watch-men? (or who will guard the guardians?): this phrase is often used these days with reference to corrupt governments, but in Juvenal's Satires the context was marital infidelity, namely that employing guards to keep an eye on or even lock up an immoral wife was pointless (as she would seduce those too, presumably).
Less common is another Latin phrase attributed to Juvenal, which has its own entry in the OED - res angusta domi, or 'narrowed circumstances at home', as in Juvenal's line "It is not easy for men to rise whose qualities are thwarted by narrowed circumstances at home". James Boswell used the phrase about Samuel Johnson in his biography: "The res angusta domi prevented him from having the advantage of a complete academical education".
Juvenal is also mentioned in the OED as having used the word 'acersecomic', which means 'a person whose hair has never been cut'.