xxx

Concise Oxford English Dictionary © 2008 Oxford University Press:
thirty/ˈθɜːti/

▶cardinal number (pl. thirties) the number equivalent to the product of three and ten; ten less than forty; 30. (Roman numeral: xxx or XXX)

– derivatives
thirtieth ordinal number,
thirtyfold adjective & adverb.

– origin OE thrītig (see three, -ty2).