I just did: the OED. Post above the Wikipedia metaphor one. It's been used in the non-legal sense for over 250 years.
If you're going to reject metaphor and semantic shift, your lexicon is going to be extremely limited.
It
was used in a none legal sense, but times change. It's not in Chambers, for example, or another dictionary I consulted.
I also think your reference to metaphors is unhelpful. They're supposed to add colour without being misleading. In my example, it's clear I didn't die making the argument. The modern interpretation of litigate is to take something to court, so using it as a metaphor for arguing doesn't work in my view. E.g. "The brute murdered him" well he didn't actually he merely stabbed him 20 times, but he lived. That's also a bad metaphor.
Anyway, it's a minor point we'll need to disagree on.