Here's why it's likely not a real phrase:
* "Ten pence to a shilling" is a real phrase, but it means something completely different. It's an old British expression meaning "almost certainly" or "very likely."
* Adding "definitely" to the phrase is redundant. The phrase itself already conveys certainty.
* "Shilling" is an old British currency. It's no longer in use, so the phrase might feel outdated or unfamiliar to younger generations.
It's possible that someone might have invented this phrase as a humorous or ironic way to express certainty. However, it's not a standard slang term with a defined meaning.
If you heard this phrase used, it's worth asking the speaker to clarify what they meant.