Is "until mine is on tight" an idiom?

I was studying English, so I downloaded a comprehension reading test and I found a phrase which I can't understand:

Seat belts save lives and that's a fact. That's why I don't drive anywhere until mine is on tight.

I think that it's an idiom because I don't find word definitions that help me understand the phrase.

Solution 1:

It's not an idiom. Each word means exactly what it says.

Mine means 'my one', referring back to the seat belt.

On tight means exactly what it says - fitted securely. So

I will not drive anywhere until my seat belt is fitted securely.

Solution 2:

" I don't drive anywhere until mine is on tight."

If you built this sentence bit by bit, it would go like this:

1. I don't drive anywhere until my belt is on.
2. I don't drive anywhere until my belt is on tight. [the belt is tight, snugly fit]
3. I don't drive anywhere until mine is on tight.

So each word (after until) means this:

• Until: while the following doesn't occur.
• Mine: reffers to [my] "seat belt" in the previous sentence.
• Is on: the belt is on = the belt is in place [i.e. the belt is fastened]
• Tight: the belt is fastened tight [i.e. the belt is tight and safety is assured].