the.com/cur

A mixed-breed dog, or an insult for a coward, same low opinion either way.

means A mongrel dog of no particular pedigree, or, applied to a person, someone contemptible and cowardly.

from Traces back to Middle English 'curre,' likely shortened from 'curdogge,' probably linked to a Scandinavian root meaning 'to growl or grumble' — the same sound a snarling, snapping mutt makes. It started as plain description of an unwanted dog lurking around a farmyard, then slid, as animal insults often do, into a slur for a human being of equally low standing.

etymology
from old norse kurra, to growl
shakespeare favorite
used it as an insult constantly
not biological term
describes mutts, not an actual breed
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