One of the weird things to happen when learning a language is when you think a word means one thing, and you find out it means something else.
For example, I bet I'm not the first immigrant to do a double take when he heard an Israeli refer to a marker as a טוש toosh. Even the most assimilated Jews in America know that tush is Yiddish for buttocks. So what's the connection?
Well, first of all, there is no connection - the words are just homonyms. Let's look at each part.
Tush comes from the Yiddish tokhes, from the Hebrew תחת tachat - bottom. The transformation does not seem to be a natural one. I found two postings on the Mendele list that gave theories as to its development.
In 1995 Zellig Bach wrote:
The accepted substitute in English for "tokhes" has been _tushie_ or_tushy_, apparently formed, according to Steinmetz, on the basis of a baby-talk diminutive. This word was then shortened to "tush," as in Mel Brooks' classic movie "Blazing Saddles," where a women bar singer by the name (I believe) of Von Push, in a satirical spoof of a Marlene Dietrich seductive song, rhymed "push" with "tush."
And in 2004, Enrique E. Gildemeister wrote:
"Nebbish" is the Western Yiddish pronunciation of "nebekh". But, could it be that, like "tush" for "tokhes", "nebbish" is used by East European immigrants' children because _kh_ does not exist in American English? Maybe the similarity with Western Yiddish is just a coincidence.
And what about the marker known as toosh? It comes from the German tusche:
NOUN: A black liquid used for drawing in lithography and as a resist in etching and silk-screen work.
ETYMOLOGY: German, back-formation from tuschen, to lay on colors, from French toucher, from Old French tochier, touchier, to touch. See touch.
The two other common terms for marker in Israel are לורד lord (originally a brand name) and מרקר mahrker - most prominently seen in the Israeli business publication TheMarker (pronounced in Hebrew as de-marker and written דהמרקר).
Another meaning of toosh in Hebrew is shower head. Another pronunciation is doosh, and according to this site, both are correct. They both originate in the European word douche, which is related to the Italian doccia meaning "shower," and is a etymological cousin of aqueduct.