septendecimal · adjective1
From Latin septendecim (“seventeen”) + -al, after decimal. By surface analysis, septen- + decimal.
septendecimal · adjective2
From Latin septendecimus + -al (after English seventeenth and decimal), from septendecim (“seventeen”), after decimus (“tenth”). Compare Latin septimusdecimus (“seventeenth”). By surface analysis, septen- + decimal. Coined by English mathematician, philologist and musician Alexander John Ellis in 1885 (see the quotation).
Etymology from Wiktionary, CC BY-SA 4.0.