"plane figure having ten sides and angles," 1630s, from Modern Latin decagonum, from Greek dekagonon, from deka "ten" (from PIE root *dekm- "ten") + gōnia "corner, angle" (from PIE root *genu- (1) "knee; angle"). Related: Decagonal.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dasa, Avestan dasa, Armenian tasn, Greek deka, Latin decem (source of Spanish diez, French dix), Old Church Slavonic deseti, Lithuanian dešimt, Old Irish deich, Breton dek, Welsh deg, Albanian djetu, Old English ten, Old High German zehan, Gothic taihun "ten."
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit janu, Avestan znum, Hittite genu "knee;" Greek gony "knee," gōnia "corner, angle;" Latin genu "knee;" Old English cneo, cneow "knee."
<a href="https://www.etymonline.com/word/decagon">Etymology of decagon by etymonline</a>
Harper, D. (n.d.). Etymology of decagon. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/decagon