Definition of pee em in US English:

pee em

noun

British
informal
  • The period after midday; afternoon or evening.

adverb

British
informal
  • Post meridiem; after midday.

Origin

1930s; earliest use found in Brevities (New York). From pee, the name of the letter P + em, the name of the letter M, after p.m. [noun¹].

Pronunciation

pee em

/ˌpiː ˈɛm/