noun
snap hairpin; triangular hairpin
A hairpin with a triangular shape that snaps shut to hold hair in place. Common in children's hairstyles.
娘の髪にパッチン留めをつけてあげた。
I put a snap hairpin in my daughter's hair.
パッチン留めがすぐに壊れてしまった。
The snap hairpin broke right away.
ヘアピン is a general term for hairpins, while パッチン留め specifically refers to the snap-type triangular hairpin.
バレッタ is a barrette, a clasp-type hair accessory, whereas パッチン留め is a snap-type hairpin.
From the onomatopoeic パッチン (snap sound) and 留め (fastener). The exact origin is uncertain, but it likely emerged as a descriptive term for this type of hairpin.