Originally: †(in the system of A. P. de Candolle): belonging to the former group Achlamydeae, comprising plants having flowers without a calyx or corolla, including willow, euphorbia, etc. (obsolete). Later: (more generally) lacking a perianth.
Mid 19th century; earliest use found in John Lindley (1799–1865), botanist and horticulturist. From scientific Latin Achlamydeae, group name (from ancient Greek ἀ- + χλαμύδ-, χλαμύς + scientific Latin -eae see -eous) + -ous.