means A slippery substance applied between surfaces to reduce friction and let them move smoothly past each other.

from From Latin lubricare, 'to make slippery,' from lubricus, 'slippery, smooth' — the same root behind 'lubricate.' That lubricus also gave us the slipperier sense of 'lubricious,' the word for someone whose intentions slide somewhere south. English picked up 'lubricant' as a noun in the early 19th century, just as the machine age made smooth-running metal a matter of survival.