Etymology Univerbation of pin + afore. The pinafore was originally pinned to the front of a dress, as it had no buttons.
1
A sleeveless dress, often similar to an apron, generally worn over other clothes, and most often worn by young girls as an overdress.
“He looks just like an angel,” said the Charity Children as they came out of the cathedral in their bright scarlet cloaks, and their clean white pinafores.1888, Oscar Wilde, “The Happy Prince”, in The Happy Prince and Other Tales:
She was a strangely thin girl in a dark pinafore, with a pink handkerchief tied over her hair.1917 September 20, Katherine Mansfield [pseudonym; Kathleen Mansfield Murry], “Feuille d’Album”, in Bliss and Other Stories, London: Constable & Company, published 1920, →OCLC, page 223:
2 more examples
The starched pinafore with the wide frills on each shoulder, which she always wore over her grey frock, was removed, and the frock itself changed for her best navy blue serge.1967, Barbara Sleigh, Jessamy, Sevenoaks, Kent: Bloomsbury, published 1993, →ISBN, page 55:
The old man heaved himself from the chair, seized Jessamy by her pinafore frill and marched her to the house.1967, Barbara Sleigh, Jessamy, Sevenoaks, Kent: Bloomsbury, published 1993, →ISBN, page 84: