If you refer to someone as a hillbilly, you are saying in a fairly rude way that you think they are uneducated and stupid because they come from the countryside.
[US, informal, disapproval]
hillbilly in British English
(ˈhɪlˌbɪlɪ)
nounWord forms: plural-lies
1. usually derogatory
an unsophisticated person, esp from the mountainous areas in the southeastern US
2. another name for country and western
Word origin
C20: from hill + Billy (the nickname)
hillbilly in American English
(ˈhɪlˌbɪli)
US
nounWord forms: pluralˈhillˌbillies
1. Informal
a person who lives in or comes from the mountains or backwoods of the South, specif. Appalachia