This is not such a common word.
Kupal is semen, specifically dried semen under the foreskin of the penis.
used to describe a foolish person
Ang kupal mo!
You’re such an idiot.
used to describe a nasty person
Kupal mo talaga!
You’re really so nasty.
Kumukupal ka na naman.
You’re being obnoxious again.
Kahit maraming kumukupal sa iyo, hayaan mo na lang.
Though are many who are being nasty to you, just let it be.