hanger-on; tagalong; clingy follower
Idiomatic expression literally meaning 'goldfish feces', used to describe a person who mindlessly follows someone around without contributing anything. Often used in a derogatory or humorous way.
He always follows the department head around like a shadow.
あの子、先輩の金魚のフンだよね。
That kid is such a tagalong to the senior, isn't she?
腰巾着 is a more old-fashioned term for a hanger-on or sycophant, literally 'waist pouch', implying someone always attached to a superior.
Common katakana spelling for the idiomatic meaning; フン is often written in katakana for emphasis or clarity.
Kanji spelling using 糞; less common but still seen.
From the image of goldfish feces trailing behind the fish; the phrase is a vivid metaphor for a person who follows another around without purpose.