I agree with falling back on opening principles. If you speculate their opening is bad, then you can also look for ways to exploit it as this video shares how to do:
Chomp on hanging pieces
...add salt and pepper as well.
Don't forget the cutlery
Follow general opening principles:
1. Pawns in the center
2. Develop Knights then Bishops
3. Castle early
4. Move your pieces once
Of course, these aren’t rules; they’re more like guidelines. Do what you judge to be best, even if it violates these principles. For example, if your opponent hangs a piece, following the “move your pieces one” rule is foolish.
The contributors are correct,if one just plays via principle, one will get a playable position; maybe it won't be the best position possible after 12 moves, but it will be one that's relatively even ( +/- 0.30).Sometimes natural-looking moves lead to a trap, in that case, unfortunately, one will just have to learn from the trap so as to avoid it in the future, this has happened to essentially almost every chess player that ever lived. Many unusual, relatively unknown openings are one-trick openings, that once you've played against them are easy to get an equal game out of.
Sometimes I play a game and my opponent plays an opening I don’t know, it’s nit weird opening but still i don’t know it,