la supposition

Simple supposition.

Just a guess.

C'est une supposition.

This is a guess.

C'est juste une supposition.

It's just a guess.

Sa supposition se révéla juste.

His guess turned out to be correct.

Pure supposition, tu n'as aucune preuve solide.

Pure speculation—you have no solid evidence.

— Ghost in the Shell (1995)

C'était ma supposition.

That was my guess.

Votre supposition est aussi bonne que la mienne.

Your guess is as good as mine.

C'est une pure supposition.

It's pure speculation.

Ce n'est qu'une supposition.

This is just a guess.

Ça n'était qu'une heureuse supposition.

It was just a lucky guess.