An old coal mining term for a young lad employed to push coal tubs from the working face to the main haulage road. Because they lacked the strength of a full-grown man, two boys would be paired up as half-marrows to share the physical labor and split the wages of a single adult putter.
Heard on the job
The overman assigned him as a half-marrow to the new boy until they were both strong enough to handle a tub alone.