The target creature looses 1/1.
LOSES
Don't want another Jester/Joker mistake.
![Tongue :P](https://elementscommunity.org/forum/Smileys/solosmileys/tongue.gif)
I think the precedent is to always use the word gain, though I don't see why lose couldn't be used, as well.
In the case of gain it would be: "Target creature gains -1/-1."