ugly customer
An especially mean, dangerous, or malicious person. I wouldn't go messing with that dude—he's one ugly customer. There are some ugly customers in this part of town, so watch your back.
ugly customer
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]