Originally (of a plant or stem): bearing a single flower spike. Later (of a clubmoss): having unbranched aerial stems.
Late 18th century; earliest use found in James Jenkinson (?1739–1808). From scientific Latin monostachyus, specific epithet (Linnaeus Mantissa Plantarum (ed. 2, 1771) 180; from mono- + ancient Greek στάχυς ear of corn, spike) + -ous.