"It's the furthest thing from my mind."
- is an expression which means:
"I'm not thinking about that. / I never think about that."
for example:
Q: How much money does he make?
A: It's the furthest thing from my mind.
---
farther - means 'more distant'.
Ex: Ottawa is farther from here than Niagara Falls.
further - means 'more time' or 'more quantity'.
Ex: The problem requires further study.
