(of a person) very careful, precise, formal, proper
When I went to his wedding, I found it strange to see him looking so prim, since he normally looked quite sloppy, in un-ironed clothes and with uncombed hair.
primus (first, finest) (Latin)
A prime minister should always dress primly In order to be very precise and formal with my speech, I had to trim all my stories. (Prim - Trim) If you're going to a PROM you better dress PRIM