-toista

Etymology
From the partitive singular of, by ellipsis of the phrase , referring to the second ten of numbers (11–19). , for example, originally was, as the first "decade" (first ten numbers) were already counted. This system used to be used more widely, but is now only used for 11 to 19, possibly by influence from the Germanic languages where those numerals too get special names, while numbers beyond 19 are more formulaic.

Suffix

 * 1) -teen suffix which forms numbers 11 to 19

Declension
Not inflected. In numbers from 11 to 19 the stem is inflected instead, see entries for those numerals.