Oily means covered in or feeling like thick liquid fat. Oily hands, an oily texture, oily fish.
Of a surface it means slick and unpleasant to touch. Of food it can be either — oily fish is a health recommendation and an oily curry is a complaint. Of a person it means falsely charming: an oily manner, oily politeness, which is the same slipperiness applied to character. Note oiliness; and that oily fish means the oil is spread through the flesh rather than stored in the liver.
1adjectiveoilyCovered in or feeling like thick liquid fat.
“His hands were oily from fixing the bike chain.”
2adjectivefalselyFalsely charming.
“The oily salesman kept calling me 'my friend' and patting my arm.”
oily is a quality of greasiness.
Oil plus -y, and German ölig and Swedish oljig are built the same way. Oil comes from the Latin for olive oil, so every oily thing is named after that fruit.
Middle English
oylei
English, today
oily
From Middle English oylei, equivalent to oil + -y. Compare German ölig (“oily”), Swedish oljig (“oily”).
Etymology from Wiktionary, CC BY-SA