Call a spade a spade
To speak plainly - to describe something as it really is.
It might be thought that this derives from the derogatory use of the slang term 'spade', meaning Negro - an American term originating in the 20th century. That view of it as derogatory might also be thought to be supported by this piece from John Trapp's Mellificium theologicum, or the marrow of many good authors, 1647:
"Gods people shall not spare to call a spade a spade, a niggard a niggard."
Trapp's use of 'niggard' is difficult to interpret. The word had several meanings in the 17th century. It could be used to mean 'miser', which is the more common usage today, or as a general term of abuse - 'lout', 'barbarian' etc. The word was also used as the name of firebricks in grates.
The co-incidence in form and pronuctiation of 'niggard' and 'nigger' causes some confusion. Although the two words probably derived independently, they doubtless affected each other's development of meaning over time.
Whatever Trapp's intention was, we can be confident that he didn't mean 'nigger' or 'negro'.
An earlier expression of the notion, albeit in different form to that which we now use, comes from Nicolas Udall's 'Apophthegmes, that is to saie, prompte saiynges. First gathered by Erasmus' - translated in 1542:
"Philippus aunswered, that the Macedonians wer feloes of no fyne witte in their termes but altogether grosse, clubbyshe, and rusticall, as they whiche had not the witte to calle a spade by any other name then a spade."
This refers back to Plutarch's Apophthegmata.
The eccentric right-wing British Tory politician Sir Gerald Nabarro was fond of emphasizing his direct 'man of the people' image by saying 'I call a spade a shovel'. In fact, despite being from an immigrant family himself, Nabarro loudly supported the repatriation of Caribbean immigrants to the UK. How he referred in private to the people who would have undoubtedly have been called 'spades' in Nabarro's social circle isn't recorded.