IPA: /ˌɒpəˈtjuːnɪst/
KK: /ˌɑːpərˈtunɪst/
A person who takes advantage of situations to gain something, often without considering ethics or the impact on others.
He is known as an opportunist, always looking for ways to benefit himself at the expense of others.
Opportunist → It is formed from "opportunus" (from Latin, meaning favorable or suitable) and the suffix "-ist" (meaning a person who practices or is concerned with something). An opportunist is a person who takes advantage of favorable circumstances to gain an advantage.
Think of someone who is always looking for 'favorable' opportunities ('opportunus') to benefit themselves — that's what an opportunist does.