Is it ensure, insure, or assure?
"pertaining to the lips," 1590s, from Medieval Latin labialis "having to do with the lips," from Latin labium "lip" (see lip (n.)). The noun meaning "a labial sound" is from 1660s. Related: Labially.
labial la·bi·al (lā'bē-əl)
Relating to the lips or labia.