noun
Refers to the taste that remains in the mouth after eating or drinking something.
このコーヒーは後口がすっきりしている。
This coffee has a clean aftertaste.
noun
Used when someone or something comes later in a sequence, such as in a queue or order of events.
The person who is later (in line) hasn't arrived yet.
Compound of 後 (あと, 'after') and 口 (くち, 'mouth'). The aftertaste sense is literal; the sequential sense is a figurative extension.