"of or pertaining to the side," early 15c., from Old French latéral (14c.) and directly from Latin lateralis "belonging to the side," from latus (genitive lateris) "the side, flank of humans or animals, lateral surface," a word of uncertain origin. Specific sense "situated on either side of the median vertical longitudinal plane of the body" [Century Dictionary] is from 1722.
As a noun, from 1630s, "a side part;" as a type of pass to the side in U.S. football, it is attested from 1934 (short for lateral pass). Related: Laterally.
Others are reading
<a href="https://www.etymonline.com/word/trilateral">Etymology of trilateral by etymonline</a>
Harper, D. (n.d.). Etymology of trilateral. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/trilateral