Is it ensure, insure, or assure?
1670s, from head (n.) in sense "heading of a book or chapter" (c.1200) + line (n.). Originally a printers' term for the line at the top of a page containing the title and page number; used of newspapers from 1890, and transferred unthinkingly to broadcast media. Headlinese "language peculiar to headlines" is from 1927. Headlines "important news" is from 1908.