It's a general enemy. Here are the first few verses, which got left out

1 My God, whom I praise,

do not remain silent,

2 for people who are wicked and deceitful have opened their mouths against me;

they have spoken against me with lying tongues.

3 With words of hatred they surround me;

they attack me without cause.

4 In return for my friendship they accuse me,

but I am a man of prayer.

5 They repay me evil for good,

and hatred for my friendship.

6 Appoint someone evil to oppose my enemy;

let an accuser stand at his right hand.

7 When he is tried, let him be found guilty,

and may his prayers condemn him.

It's not talking about a specific person, but enemies in general.

It's a psalm that wishes bad on those who oppose you and the love you give. Basically a disstrack on all your enemies.

Wikipedia says it's attributed to King David but doesn't give more context. Maybe cursing someone that betrayed him?