Not round. In later use specifically (in Phonetics): not enunciated with the lips rounded.
Late 16th century; earliest use found in Cyprian Lucar (1544–?1611). From un- + round.
with object To make (something) no longer round. In later use specifically (in Phonetics): to reduce or eliminate the rounding of the lips when enunciating (a sound, especially a vowel).
Early 17th century; earliest use found in Randle Cotgrave (fl. 1587–?1630), lexicographer. From un- + round, originally after French †desrondir.