Definition:
A symbol of gratitude, sometimes affection.
Usage:
"I'll give you a yankee dime if you'll go inside and fetch me a glass of iced tea."
Explanation:
I imagine the term came into usage following the American Civil War, after Confederate currency was rendered worthless, and Federal currency came to represent real value. Considering inflation, a "yankee dime" would have been a nice reward for a small task, and likely hard to come by for the speaker. An offer of a "yankee dime" is sort of similar to the contemporary "I'll be your best friend if . . ." trope.
Trivia:
In my Virginia-based extended family, a "yankee dime" is sometimes used a shorthand for a kiss.
