Interestingly, unctuous is derived from the Latin unctus which means "anointed with oil," which is where the "oily" connotation comes from. Unctuous and "oily" are synonyms that both suggest that someone is trying to butter you up; they're being nice, because they're hoping you'll give them what they want. Talk-show hosts, used-car salesmen, people who want your job: all of these are people we think of (rightly or wrongly) as being unctuous. Being an unctuous jerk, he gave me a gift, hoping I'd give him a record deal.