'tagalong' originates from American English, formed from the phrasal verb 'tag along' (early 20th century), where 'tag' carried the sense 'to follow or stick to' (informal).

'tag along' likely developed from the verb 'tag' (informal use meaning 'touch, mark, or follow') combined with 'along' to indicate accompanying; the compound 'tagalong' later emerged as a noun and adjective in colloquial use.

Initially used informally to describe the action 'to follow along'; over time it became a noun/adjective ('a tagalong') with a slightly negative sense of an unwelcome or dependent companion.