Learner's definition of OUNCE
a unit of weight equal to ¹/₁₆ pound (about 28 grams)
a very small amount of something
If you had an ounce of common sense, you wouldn't try it.
He doesn't have an ounce of decency.
She doesn't have an ounce of fat on her body. [=she is very thin]
That story doesn't have an ounce of truth in it. [=that story is completely untrue]