S-o Definition
letter
numeral
The ordinal number nineteenth, derived from this letter of the English alphabet, called ess and written in the Latin script.
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noun
American Library Association. Abbreviation for sextodecimo, a book size, 15-17.5 cm in height.
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abbreviation
(informal) Significant other.
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(chiefly grammar and lexicography) Someone.
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Wiktionary