a hostile or dangerous person.
Origin of ugly customer
First recorded in 1805–15
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
An ill-natured or vicious individual, as in Watch out for Charlie when he's drinking; he can be an ugly customer. This phrase uses ugly in the sense of “mean” or “dangerous.” [c. 1800]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.