udder

[noun]
[de ui-er, de ui-ers]

"Uiers" are the mammary glands of ruminants: mammals that re-chew their plant-based food before further processing. The most well-known Dutch animal with "uiers" is of course a "koe": a cow  

Sometimes "uiers" are used to refer to a woman’s (large) breasts, but this is strongly sexist and derogatory.

"Uier" has nothing to do with "ui": onion…

Examples:
- "Een koe produceert melk via de uier." 
("A cow produces milk via the udder.")

- "Ken jij nog meer dieren met uiers?" – "Jazeker: geiten en schapen hebben ook uiers." 
("Do you know more animals with udders?" -"Sure: goats and sheep have udders too.")

- "Anita is afgestudeerd op de invloed van uierbehandeling op de kwaliteit van melk." -"Te gek! Dat wist ik niet!" 
("Anita has graduated on the influence of udder care on milk quality." – "Great! I didn’t know that!")

Expressions:
- "Melk is goed voor elk": (Dutch rhyme) milk is good for everybody.

Related words:
- Melk: milk [noun] [de melk, <no plural>].
- Melken: to milk [verb] [melken, molk, h. gemolken].

Example:
- "Koeien worden tegenwoordig niet meer door de boer zelf maar door een machine gemolken."
("Nowadays, cows are no longer milked by the farmer himself, but by a machine.")

- Borst: 1. chest. 2. breast [noun] [de borst, de borsten].
- Koe: cow [noun] [de koe, de koeien].
- Schaap: sheep [noun] [het schaap, de schapen].
- Geit: goat [noun] [de geit, de geiten].

Extra:
For more information on Dutch dairy farming, check out the (Dutch) website of "Productschap Zuivel", literally: productship dairy. A "productschap" is a public organization that represents businesses that process one product/resource in different phases of the product chain.