To put it bluntly, to be outspoken; to speak the truth, to say things as they really are.

Etymology

A mistaken translation of Ancient Greek τὰ σῦκα σῦκα, τὴν σκάφην δὲ σκάφην ὀνομάσων (tà sûka sûka, tḕn skáphēn dè skáphēn onomásōn, “calling figs figs, and a trough a trough”). The word σκάφη (skáphē, “trough”) was mistranslated by the medieval scholar Desiderius Erasmus as σκαφείον (skapheíon, “digging tool”).

Note

The usage of "spade" in this idiom originally meant the garden tool. However, in modern usage, the word "spade" has been used as a derogatory and racist slur against black people. See the following NPR article for more info.

Sources

Wiktionary