If you meant the meaning of the Kanji 志保 , then I don't know =_= I just know that the name "Shiho" might come from "Sherlock Holmes"...
Anyway, has anyone ever wondered why Ai's parents were given such nicknames ? You know, "Mad Scientist" and "Hell Angel" ? "Mad Scientist" is common in fictions, so I don't see any specialty. "Hell Angel", on the other hand, sounds much creepier. Not Lucifer/Satan/Demon/Devil... It's "Hell Angel". Despite the fact that she seems to have a nice personality.
Haibara once described that Elena was known in the Org. as the kind of person "who didn't talk much and acted mysteriously, nobody knew what she was thinking" (Vol 41). That doesn't tell us a thing, many people in the BO also have such personality, not to mention that it is not a suitable thing to call mysterious people. To be called something like "Hell Angel", you need to have a high "evil-degree"...
I've read somewhere in DCTP that the nickname "Hell Angel" of Elena might be given by Vermouth (you know, the "Angel" part...) And it somehow got spread widely within the Organization. Although I'm pretty sure that it was just a behind-the-back-nickname, I think Elena was aware of this...
There could be many reasons why she has got that nickname:
1. Her research was going "against the stream of time", or in other words, "against the will of God", which Vermouth doesn't seem to like. "Hell Angel" probably means Fallen Angel. This theory is the most likely in my opinion, and many people has also come up with it.
2. If the Vermouth theory was correct, then Elena might have been Vermouth's "Angel" in the past (before she met Ran). Elena might be the person whom Vermouth once looked up to, but she ended up betraying her. She might have taken advantage of Vermouth and used her as a guinea-pig for the experiments. Therefore Vermouth held a grudge against her and named her "Hell Angel", which was even worse than "Fallen Angel". But since Sharon has already stated: "No angel has smiled upon me. Not even once.", this theory is rather unlikely.
3. If Elena is actually Vermouth (which I find unlikely), then she is referring to herself as "Hell Angel". Which means that Vermouth might have been a good person in the past (Angel), but for some reasons she "fell" and ended up in "Hell" (BO). This explains why she is always waiting for a "silver bullet" to come and destroy the Org.
P/S: Edited.