forty

Concise Oxford English Dictionary © 2008 Oxford University Press:
forty/ˈfɔːti/

▶cardinal number (pl. forties)

• 1 the number equivalent to the product of four and ten; ten less than fifty; 40. (Roman numeral: xl or XL)

• 2 (the Forties) the central North Sea between Scotland and southern Norway, so called from its prevailing depth of forty fathoms or more.

– phrases
forty winks informal a short daytime sleep.

– derivatives
fortieth ordinal number,
fortyfold adjective & adverb.

– origin OE fēowertig (see four, -ty2).