surname, early 13c., probably an agent noun from walk (v.) in the sense "to full cloth." preserves the cloth-fulling sense (walker with this meaning is attested from c. 1300). "Walker" or "Hookey Walker" was a common slang retort of incredulity in early and mid-19c. London, for which "Various problematic explanations have been offered" [Century Dictionary].

"Is it?" said Scrooge. "Go and buy it."

"Walk-ER!" exclaimed the boy.

"No, no," said Scrooge. "I am in earnest" (etc.)

[Dickens, "A Christmas Carol"]