fem. proper name, shortened form of Gillian. Also see Jill. Gill-flirt "giddy young woman" is from 1630s.

gill (n.1)

"organ of breathing in fishes," early 14c., of unknown origin, perhaps related to Scandinavian words, such as Old Norse gjölnar which perhaps means "gills," and Old Danish -gæln (in fiske-gæln "fish gill"); said to be ultimately from a PIE *ghel-una- "jaw" (cognate: Greek kheilos "lip"). Related: Gills.

gill (n.2)

liquid measure (commonly a half-pint), late 13c., from Old French gille, a wine measure, and from Medieval Latin gillo "earthenware jar," words of uncertain origin, perhaps related to the source of gallon.