OS

PRONUNCIATION: (aws)

MEANING: noun:
1. A mouth or an orifice. [plural ora]
2. A bone. [plural ossa]

ETYMOLOGY:
For 1: From Latin os (mouth). Earliest documented use: 1859.
For 2: From Latin os (bone). Earliest documented use: 1400.

NOTES:
It also appears as an abbreviation in many fields, including
Chemistry: Os - symbol for the element osmium
Computing: OS - Operating System
Medicine: OS - left eye (from Latin oculus sinister)
Linguistics: OS - Old Saxon

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OZ:
1. a surgeon who got a TV show and started endorsing products of dubious value
2. abbreviation for "ounce"
3. magical land, site of many L. Frank Baum stories
4. vernacular for "Australia"

ON:
1. opposite of "off"
2. a debt of honor, in Japan

OB:
1. a physician who practices Obstetrics, delivering babies
2. an obligation (see ON above)