Late 19th century; earliest use found in The Century Dictionary. Probably originally from scientific Latin Monocondylia (Haeckel Gen. Morphologie der Organismen II. p. cxxxii; from ancient Greek μονοκόνδυλος having only one joint + scientific Latin -ia) + -an; compare German Monocondylien (Haeckel 1866, as above). Probably subsequently re-formed from mono- + condyle + -ian. Compare monocondylar and slightly later monocondylic.