Lower, Mark A (1860) Patronymica Britannica: a dictionary of the family names of the United Kingdom. London: J.R. Smith. Public Domain.
SHADBOLT
As described in historical dictionaries
Patronymica Britannica (1860)
SHADBOLT. Ferguson derives this name from Northern roots, signifying bold in war. I think the last syllable is more likely to be a corruption of the local hold., a dwelling.