The Collaborative International Dictionary
hyporachis
Hyporhachis \Hy`po*rha"chis\, n.; pl. Hyporhachides. [NL., fr. Gr. "ypo` beneath + ? spine.] (Zo["o]l.) The stem of an aftershaft or hypoptilum. [Written also hyporachis.]
Wiktionary
hyporachis
n. (context ornithology English) The downy lower barbs of a feather.