Pallida mors - Pale Death. (Horace)
Palmam qui meruit ferat - Let him who has earned it bear the reward
Panem et circenses - Bread and circuses. Food and games to keep people happy. (Juvenalis)

Par pare refero

I return like for like tit for tat retaliation

Parens patriae - Parent of the country
Pares cvm paribvs - Like persons with like persons. Birds of a feather flock together
Pari passu - With equal pace - moving together
Pars maior lacrimas ridet et intus habet - You smile at your tears but have them in your heart. (Martialis)
Particeps criminis - Partner in crime
Parturient montes, nascetur ridiculus mus - Mountains will be in labour, and an absurd mouse will be born. (all that work and nothing to show for it)