。
I'm not happy at all.
Literal
I [topic-は] even-a-little happy [copula-じゃ] not.
少しも~ない ('not even a little, not at all') is a total negation pattern. The quantifier 少し combined with も and negative means 'not even the smallest amount.' Similar to 全然~ない but emphasizes the minimal quantity denied. じゃない is the casual contraction of ではない.