adulterous
Concise Oxford English Dictionary © 2008 Oxford University Press:
adultery/əˈdʌltəri/
▶noun voluntary sexual intercourse between a married person and a person who is not their husband or wife.
– derivatives
adulterous adjective,
adulterously adverb.
adulterous adjective,
adulterously adverb.
– origin C15: from obs. adulter, from L. adulter ‘adulterer’.
'adulterous' also found in these Oxford entries:

