From theGerber Childrenswear brand nameOnesies. The name becamegeneric and was reinterpreted as a plural, hence the new singular anddecapitalized "onesie". Equivalent toone +-sie.[1]
2023 February 2, Natalia Winkelman, “‘Baby Ruby’ Review: Enfant Terrible”, inThe New York Times[1]:
It was once considered taboo to even suggest that new motherhood was not all sunshineonesies and rainbow mobiles, and “Baby Ruby” arrives on a welcome wave of contemporary movies exploring how the joys of child rearing can commingle with misery.
2024 August 16, Natasha Dye, “Caitlin Clark Signs Indiana Fever Fan's 2-Week-Old Baby: 'So Cute'”, inPeople[2]:
The Fever shared a video of the meeting on X (formerly known as Twitter), where an excited Clark got to hold the young baby and sign the youngster'sonesie.