Mid 19th century; earliest use found in Miles Berkeley (1803–1889), Church of England clergyman and naturalist. Either from pycnide (or its etymon French pycnide) + -ium, after Latin nouns with this ending, or perhaps from scientific Latin pycnidium (attributed to L. R. Tulasne by Robert Dict. alphabétique et analogique, though this use has not been traced) from ancient Greek πυκνός thick, dense + scientific Latin -idium.