spencer · noun1
From the family name Spencer. The jacket is probably named after George Spencer, 2nd Earl Spencer (1758-1834).
Spencer · proper noun2
An occupational surname from Middle English Spenser, Spencer, Spensier, from the common noun spenser (“spencer”), denoting someone who works in a spense (“spence”), or as a butler or steward.
Etymology from Wiktionary, CC BY-SA 4.0.