Pain in the coccyx (tailbone) at the base of the spine, often resulting from injury, prolonged sitting, or childbirth.
From Greek 'kokkyx' (cuckoo bird, later used for tailbone because of its shape) + 'dynia' (pain). The medical term emerged in English medical literature in the 19th century.
Coccydynia became almost epidemic after the rise of office work and driving—the coccyx evolved for different body positions in our ancestors, so modern sitting for 8+ hours creates pain it never encountered in human history.
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