Merriam-Webster's Advanced Learner's Dictionary:
forty
for·ty /ˈfoɚti/ noun, pl for·ties
1 [count] : the number 40
2 forties [plural]
a : the numbers ranging from 40 to 49
• The temperature outside is in the high forties.
b : a set of years ending in digits ranging from 40 to 49
• She is in her forties.
• She found some old record albums from the forties. [=from 1940–1949]
- for·ti·eth /ˈfoɚtijəɵ/ noun, pl -eths [count]
• one fortieth of the total
- fortieth adj
• his fortieth birthday
- forty adj
• forty days
- forty pronoun
• forty of her classmates
- for·ty·ish /ˈfoɚtijɪʃ/ adj
• He was tall, fortyish [=about 40 years old], and had glasses and brown hair.
