A combination of the words Yiddish and English. It refers to the use of words or expressions in American English that were originally of Yiddish origin. Yinglish is especially common among Jews from Eastern Europe and New Yorkers (because of the high Jewish population). The combination of the two words implies that the words being used are not quite those found in traditional Yiddish, but rather an English version of a traditional Yiddish words, phrase, or saying.
In Yiddish, this meant "nerve...presumption-plus-arrogance" (according to the Joy of Yiddish by Leo Rosten) Implying, he's got a lot of nerve to say that crap to me.
In Yinglish, however, chutzpah has come to mean spunk or guts in a more positive sense. Might be used to describe someone who dared to be brave enough to do something.
When a person from Great Britain lives in America and adopts a phonyAmerican accent. Other possibilities are Scotlish (adding aye at the end of every sentence doesn't make you Scottish!), Welsh (cos it's made up)!
Son you were born is Wales, and I know you live in America, but how come you sound Yanklish!