a silly fellow; a stupid, foolish person; a simpleton; a giddy or flighty young girl ...1589 obs.
from goose (n. a foolish person, a simpleton) + cap (n. the head - used in combinations, such as fuddle-cap, etc.)
FIRST DOCUMENTED USE
1589 - see EXAMPLE below
"...And so will you Sonnes both, like a couple of goosecaps, (if you looke not to it) as your father did, with your own goose quil..."
From: Martins Months Minde that is, a Certaine Report, and True Description of the Death, and Funeralls, of Olde Martin Marreprelate, the Great Makebate of England, and Father of the Factious
- Marphoreus (Thomas Nash)