What makes "sobre" a masculine noun?

Question

what mkes sobre a masculine noun?

Answer 1

Do you mean sobre when the word names an object like an envelope?

sobre - RAE

Are you asking why it is masculine-perhaps because of its ending in bre?

Now that the dictionary has a wildcard feature go there and enter *bre and see the long list that you get. Most if not all, I didn't look at the entire list, are masculine words, (e.g. hombre, timbre, nombre, hambre, sabre, fibre) fiebre (feminine)

I like it when they put words like fibre (fiber) in the list, but then when you go to look it up, it says there is no such word, it's fibra)