"Biddy" is actually a very interesting word because it has two separate origins, both fairly well-documented, which is unusual for a slang term. The primary meaning of "biddy" is "chicken," and it first appeared in the early 17th century. The word probably came from the nonsense syllables used to call chickens -- something like "here biddybiddybiddy," I suppose. By the late 18th century "biddy" had been adopted as a derogatory slang term for women, much in the same unfortunate way that "chick" was in the 1960's.

However, "biddy" in this sense might have died a welcome death had it not been for the influx of Irish immigrants into the U.S. in the early 19th century. Young Irish women often had their passage paid by upper-class American families, for whom they would then work as domestic servants while they paid off their debt. The practice was so widespread that such women came to be known as "Biddies," from a shortening of "Bridget," a common Irish women's name. This use of "biddie" reinvigorated the word, and ever since it has been employed by insolent children to torment their elders.
Sometime in the last 20 years or so, the term was recycled to mean a woman of any age. The connotation may exemplify the earliest expression synonymous with the concept now known as MILF.
"The old biddy is playing bridge"

or

"I'm trying to pick up a biddy"
by skull_dugger December 1, 2007
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In Australia a biddy is a two-for-one McDonald's voucher. It usually entitles the bearer to enjoy two delicious Big Macs for the price of one, but can apply to other items, such as fries and cheeseburgers.
Hey Cletis, some man in the street gave me a biddy! Looks like the whole family will be eating tonight!
by Yddib September 6, 2010
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In Australia, a biddy is a really cool and attractive person who appreciates 2-for-1 McDonald's vouchers
Hey biddy, I got given a biddy today - you couldn't get more value if you tried!
by sexybetchfromadelaide September 7, 2010
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A "biddy" is one of those girls who wears very short skirts and very high heels in very cold weather, and walk around downtown areas on their way to parties where they're going to get smashed and act like hoes, and then make the walk of shame back to thieir dorms in the same heels and short skirts the next morning.
My favorite past time is definitely biddy watching. I always wait for one of those girls to trip over their rediculous high heels and fall into a pile of snow. Too much fun.
by credittok.plante January 30, 2011
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In Britain it means an old lady, completely opposite to other such definitions.
by spudcake December 20, 2004
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Hit of LSD.
I just took a biddie man and I feel like i'm flying.
by Legs 35 June 21, 2010
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