lintel (n.)

"horizontal piece resting on the jambs of a door or window," early 14c., from Old French lintel "threshold" (13c., Modern French linteau), a word of uncertain origin, probably a variant of lintier, from Vulgar Latin *limitalis "threshold," or a similar unrecorded word, from Latin limitaris (adj.) "that is on the border," from limes (genitive limitis) "border, boundary" (see limit (n.)). Altered by influence of Latin limen "threshold."