Webster1913's definition is now obsolete -- it doesn't even appear in most dictionaries of the past 30 years. In fact, if you have an old dictionary, there's a good chance it wont have any yuck in it at all. But around 1965, yuck suddenly reappeared, this time as an informal American slang term with a completely different meaning; yuck is now an exclamation of disgust. "Yuck! Don't eat that!".
Yuck may also be used as a noun "It's full of yuck", or an adjective, "That's yuck!, although the more common adjective form is 'yucky'.
This second, slangish form of yuck is now common enough that it is working its way back into dictionaries (and spell checkers), and is used in English speaking countries the world over (if you live in an English speaking country that does not use yuck, let me know!), although it is still regarded as informal, and perhaps even childish. We aren't really sure where this second meaning of yuck comes from; most sources guess that it is echoic, just like ick, bleh, and blech. Www.etymonline.com notes that Newfoundlanders were using yuck back in 1963 to refer to vomiting.
Yuck may also be spelled yuk, although the usual convention is that 'yuck' means icky and 'yuk' refers to a laugh ("Yuk it up"; yuk, in this sense, may also be spelled yuck, yock, yok, yak, and nyuk.) Variations on yuck such as yeeeck! and yech! are also sometimes used for extra emphasis.
Are you Japanese? Http://www.takeourword.com/et_t-z.html#yuck hints that yuck (possibly in the form of 'yach' or 'yack') may be of Japanese origin, but sadly, I can not find any other sources to confirm this. Can you help?