This is the 1960th most frequent Japanese word.
こいつ
"こいつ" means "this guy," "this person," or "this thing" (casual/slang; can be rude depending on context).
こいつ is used informally to refer to a person in a slightly rude or joking manner.
こいつはいつも遅刻する。
This guy is always late.
Here, こいつ is used with a dismissive tone, suggesting the subject is not worth engaging with.
こいつに話しても無駄だ。
Speaking to this person is pointless.
こいつ is used casually to refer to someone in a familiar and somewhat affectionate manner.