C18: from Latin undecim eleven (from unus one + decem ten) + -gon
undecagon in American English
(unˈdekəˌɡɑn)
noun
a polygon having 11 angles and 11 sides
Word origin
[1720–30; ‹ L un-, as in undecim eleven (ūn(us) one + -decim, comb. form of decemten) + decagon]This word is first recorded in the period 1720–30. Other words that entered Englishat around the same time include: arrangement, berm, catchword, deadwood, personify