Oily describes something covered with, containing, or feeling like oil. When you touch a greasy pizza slice or a car engine, that slippery, slick feeling on your fingers is oiliness. Foods like french fries, potato chips, and bacon are naturally oily. Some people's skin produces more oil than others, making their faces look shiny by the end of the day.

The word also describes someone whose behavior feels slick and insincere, like they're trying too hard to be charming or persuasive. An oily salesperson might compliment you excessively while trying to sell you something you don't need. When someone speaks in an oily tone, their voice sounds artificially smooth and friendly in a way that makes you suspicious of their real intentions.

You can also use oily to describe liquids that behave like oil: thick, smooth, and resistant to mixing with water. An oily substance leaves residue on surfaces and requires soap to wash away properly, since water alone just beads up and rolls off.