As much as I think it's worthy to reconstruct protolanguages and even to think seriously about long-range comparative linguistics, I'm not in favour of loose and ill-informed etymologies. This is one of those badly reconstructed items that needs to be undone since it's a fact that Proto-Semitic (PSem) *sabʕatum is reconstructed as a valid, masculine form of the numeral "seven" and it's also a fact that Proto-Indo-European (PIE) "seven" is *septm̥. It's self-evident that Indo-European had borrowed this word from Proto-Semitic peoples sometime before 4000 BCE since the word in Indo-European is unanalysable while in Proto-Semitic, the word is known to be formed from the numeric root *sabʕ- This speaks volumes about a widespread neolithic trade between speakers of both Indo-European and Semitic languages, not to mention a network of other protolanguages.
However, little else is said about this and IEists seem too busy with other things than to explore how the Pre-Indo-European dialects sat within the context of prehistoric society and the budding Mediterranean economy of the 7th and 6th millenia BCE. I seem to be one of the few even mentioning any of this at all. And I can't fathom why because it's a fascinating story.
Well, it turns out that our Jewish siblings at Forward are also aware of this fun linguistic factoid. They offered a funny article in December 2005 about the false Hebrew etymology for the word "British" in Is British Ish Brit?. After explaining how real linguistics words and renouncing these eyeball etymologies, they then mention:
"Yet even the invention of wine came at a relatively late stage of human development. There are some Hebrew-English word connections that may go back further. Take, for example, Hebrew sheva, “seven”: While its resemblance to the English numeral might appear a pure coincidence, there are reputable linguists who think that it isn’t and that sheva and seven’s oldest ancestors, conjectured Proto-Semitic sab`atum and conjectured Proto-Indo-European septm, may owe their similarity to contact as much as 10,000 years ago between early Indo-European and Semitic speakers in or near the eastern Mediterranean."Yep, and it's very correct. For those not caught up on the subject of neolithic languages, you may be forgiven if your mind may stray and start envisioning a crazy dialogue between Indo-European and Semitic traders:
Indo-European guy: So tell me, Ariel, do you have the bottles of wine my village requested?Putting jokes aside, Indo-European speakers were undoubtedly a varied lot of people, differing from region to region in appearance, customs and dialect. Same too for Proto-Semitic speakers. Somewhere in the middle they met.
Semitic guy: Yes, Hans, right on schedule. Straight from a village in Turkey. But oy! Was it ever a pain in the tokhes sailing down the Danube today. The winds were blowing and my sail almost broke off. Took me almost sab`atum days to get here just from one of your local villages upsteam! But listen to me, don't I sound like a pitshetsh. Forgive me, how was your day?
Indo-European guy: Say, what was that you said? Septm, what does that mean?
Semitic guy: Oh, I said, sab`atum, you know, the numeral after... how you say... sweks?
Indo-European guy: Ah yes! Hahaha. You know, Ariel, your word is so much better than our word. I'm tired of saying "five plus two" all the time, so from henceforth I shall say septm like you.
Semitic guy: Haha, wonderful, however you're pronouncing it all wrong. What's with you crazy Indo-Europeans and your inability to pronounce a simple pharyngeal?
Indo-European guy: Haha, let's talk more over a cup of medhu.
Semitic guy: Say what now?
And now to offer my theoretical explanation of events that transpired. My personal suspicion is that a fair amount of Proto-Semitic (PSem) vocabulary was adapted into Pre-Indo-European sometime between 6000 and 5500 BCE because of a network of neolithic trade going on. The Semitic numeral for "seven" would have had stress accent on the first syllable: *sábʕatum. It was then adopted into Pre-Indo-European (Pre-IE) as *séptam. You see, being that Pre-IE speakers didn't have pharyngeals in their language, the sequence -bʕ- just sounded like a strange-sounding "p". The second "a", being unstressed in PSem, was probably pronounced as a short schwa and thus explicably inaudible to some Indo-European ears. PSem *a would have been a front vowel like the "a" in "cat" in order to explain its change to *e in Pre-IE.
Over time, the stress accent in Pre-IE eroded unstressed syllables, producing *séptm̥. This later changed to *septm̥ when the accent shifted to the last syllable to match the accent pattern of the neighbouring numeral *oḱtō "eight". This explains why a reduced syllable with only a nasal vowel came to have accent. Yet then why did they borrow the numeral and why is this same Semitic numeral also borrowed in other protolanguages like Proto-Kartvelian? I think this was simply a matter of religious connections that are lost to us. Later Babylonians associated certain numbers with deities and I suspect that a precedent existed in the neolithic, revolving particularly around the numerals "six" and "seven". These numbers must have symbolized someone or something that was widely thought to be important enough to extend beyond the many cultures and languages of Eastern Europe and the Near East. So this is why I think that the significance of these numerals was a matter of early religious beliefs.
So when you consider the potential for many of these Indo-Europeans to have been borrowed during the neolithic from Proto-Semitic, it makes many of Bomhard's reconstructions questionable, particularly those based solely on data from Proto-Semitic and Proto-Indo-European. On the other hand, I wonder how many of Bomhard's "Nostratic cognates" could be reinterpreted as evidence for Semitic loans in IE. These are all things that make me go hmmmm...
 Proto-Kartvelian *šwid- "seven". According to Klimov (Etymological Dictionary of the Kartvelian Languages, page 251), "a closer relationship of Akk. šibit 'seven' is quite evident".