One of the Israeli prime ministers was strongly criticized for saying Shmorocco in a debate: and he would have been speaking Hebrew rather than Yiddish, so either it has been taken up into Modern Hebrew from Yiddish, or it was Hebrew to begin with.

I don't know of any basis for it in Ancient Hebrew. That is, I can't think of a grammatical explanation for it: it looks like a sound-symbolic thing. There are so many variously derogatory terms in sh + consonant (shlemiel, shnook, shlepper, shlemozzle, shnozzola, shmuck, shmoe, etc. etc.) that it looks like a prefix. I don't know enough Hebrew to see whether there's any common pattern in forming these.

One thing. The Biblical relative pronoun is 'asher, but there was apparently a short form sh. I've never seen it in the Bible so perhaps it was a later formation, a contraction of 'asher. In Modern Hebrew it forms a possessive 'of' when combined with l- 'to': i.e. the prefix shl- 'of' comes from 'which-is-to-'. This suggests that the sound-symbolic sh- forms might have arisen in a similar way.

Afterthought. I gather Begin (or whoever it was) just said Shmorocco instead of Morocco, i.e. no reduplication. It was this departure from the familiar Yiddish idiom that attracted my attention.