be worth one's keep or salt. Work well enough to deserve what one is paid, as in Get a job—it's time you earned your keep, or With that batting average he's not worth his salt. The keep in this phrase refers to “room and board,” which in former times sometimes constituted the only reward for working (on a farm, in a home, etc.). The salt stands for “salary” and alludes to the ancient Roman practice of paying soldiers an allowance to buy salt.
[ First half of 1800s