means Covered in, containing, or resembling oil — slick to the touch, or by extension, smoothly insincere in manner.

from From 'oil' plus the adjective-forming '-y,' the same ending that turns 'dirt' into 'dirty.' 'Oil' itself rode into English through Old French 'oile,' from Latin 'oleum,' meaning olive oil — which traces back to Latin 'olea,' the olive tree, and a Greek cousin 'elaia.' So every greasy gear and slippery salesman owes a debt to the Mediterranean olive grove. The figurative 'oily' — meaning falsely smooth or unctuous — is centuries old, born from the simple observation that anything coated in oil is hard to grip and hard to trust.