Pronounced like "gan-zee"
Literally, this is the Irish word for jumper (which is the UK/Irish term for a sweater).
Used as an insult to imply that somebody has the intelligence of the aforementioned garment i.e. none at all
Louise: "Let's go shopping!"
Steven: "It's midnight, you're drunk, and we're in the pub. The shops are closed! You big geansaí!"