Old English stīweard joined a word for a hall or sty with weard, a guard or keeper. English steward retains the idea of being entrusted with care or management.
The source gives stī and stig as alternatives. The puzzle does not narrow the history to a specific pig-keeping job.
Sources
- Merriam-Webster: steward
Noun Word History and Kids Etymology give the compound and the hall/sty and keeper glosses.
- Online Etymology Dictionary: lord
Also built on *weard*: lord, from Old English hlaford, earlier hlafweard (hlaf "loaf" + weard "keeper, guardian")