yleisimmä
Yleisimmä is not an independent word in Finnish but a morphological stem form related to the adjective yleinen ("common"). It represents the superlative stem of that adjective (from superlative yleisin, "most common") and appears when the superlative is inflected with case endings or combined with other suffixes. Linguistically it is often written with a hyphen as yleisimmä- to mark that it is a bound stem rather than a standalone lexeme.
The form appears in inflected variants such as yleisimmän (genitive singular, "of the most common"), yleisimmässä
Etymologically it derives from the adjective yleinen with the comparative/superlative morphology of Finnish; the superlative stem