It is not a loanword, it is a relative word. As a speaker of both, I know that the two languages have many similarities but slim to nothing from Hebrew is a loanword from Arabic. Remember, Hebrew is a language MUCH older than the Arabic language, regardless of its modernization. Regardless, it is good to use Arabic as a reference to learn Hebrew because of the similarities. I myself am learning Iraqi Arabic and my friends speak south Yemeni Arabic and I find some of the similarities striking.
It is a loan word. Ancient Hebrew had, like most ancient languages, only about 10,000 root words. Modern Hebrew several times that, just like other modern languages. Most of the new words were derived from existing Hebrew roots, but many were borrowed from Arabic, Latin, Greek, Yiddish and even English. כיף comes from كَيْف (kayf), which is an ancient word from the Arabic root k-y-f.
One good way to identify a probable root is to look at the definition. If it only means one or two things in Hebrew, but a cognate in another language has multiple meanings, the Hebrew probably borrowed from the other language, or perhaps from a middle man. This is because native Hebrew words often have multiple meanings, just as native Arabic words do, while loan words are usually only borrowed for one or two meanings.
Another way is to try to find how long it's been used in each language. Seek, but you shall not find כיף in ancient Hebrew.
Also note that spoken Arabic is much older than written Arabic, and that Arabic names are found in Akkadian inscriptions on the Kurkh Monoliths describing the reign of Shalmaneser III down to 853 BC. Later Nabataean inscriptions with numerous Arabic names date from the 2nd century BC.
Well, what I was saying is sometimes a consonant that would have the Tzere niqqud is followed by yod to guide pronunciation in general writing (which lacks niqqud).
In both Arabic and Tiberian Hebrew, כיף is pronounced keif/kayf (it gets the Tzere under it). I am not familiar with modern Hebrew colloquial pronunciation or with Ashkenazi vocalization, but only Sephardic and somewhat Yemeni and Tiberian. However, after seeing your comment I looked it up and it does seem Tzere is pronounced the same as segol in modern Hebrew. I guess even a language that was dead for centuries isn't immune to being simplified over time. ;)
Just looked up the pronunciation in Tiberian Hebrew, it's quite interesting (I remember encountering this a long time ago when editing Wikipedia), but that's not really what I was trying to say.
Basically in Hebrew, the letters א, ה, ו, י can be used both as consonants and as vowels. In this case, י is a vowel. That is, it does not have any nikud at all. Therefore its "pronunciation" is silent. As far as I know, this applies to all forms of Hebrew since nikud was invented.
Without seeing it for myself I'm not quite sure what you're referring to. If, as I suspect, you mean a dictionary hint in the Latin alphabet, then it probably is designating the "singular" as that's the standard abbreviation. If a choice in Hebrew such as סג or שג then I have no idea.
Sorry, to me it showed in the notification that you wrote what I wrote. Did you perhaps edit the question?
As far as the difference between "no fun" and "not fun", I agree. You should report it if you come back to that sentence and they can add the additional translation.