A fourty is a beer that comes in a fourty ounce bottle.
Get me a fourty at the liquer store.
A very common misspelling of "forty".
<resu1> Ooh, an online chat room. Can I join?
<nimda> Uhh... sure.
<resu1> You know, I just played fourty straight hours of Warcraft last night.
<nimda> What'd you say?
<resu1> I haven't slept for fourty hours because of that Warcraft session.
<resu1> Fourty, yeah.
<nimda> You mean forty.
<resu1> Yeah, fourty.
<nimda> You're saying it weird, why are you putting so much emphasis on the "u"?
<resu1> What're you talking about? I'm just saying it. Fourty. There are fourty days in Lent. Four times ten is fourty.
<nimda> Say "four".
<nimda> Now say "forty".
<nimda> You're eating hair!
<dernston> Yeah, really badly done "Stewie and Brian" parody sketch there.