An informal or archaic shortened form of 'although,' used to introduce a contrasting or conceding statement.
A colloquial abbreviation of although that became popular in informal writing and speech, though it was never standard in formal English. Early English combined 'all' and 'though' into 'although,' which speakers then shortened to 'altho'.
Text messaging isn't new—people wrote 'altho' in personal letters and postcards to save writing space over 150 years ago, and 'tho'' is still recognized enough that some style guides permit it in very casual contexts.
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