The motto calls upon connotations and associations in Latin that are hard to evoke in an analogous way in English. So here is a clumsy translation followed by some exposition of generosus and virtus so you can follow for yourself how they combine in the motto:
High-born manliness fears nothing.
The adjective generosus = genus + -osus. Genus is your birth, your origin, your kind—like gens, your clan. If you are genere Anglus, then you are an Englishman by birth, that is, originally, not as an immigrant. Genus is the root of a great many words in English, like "generate", "general", and "generic". These all grow out of a metaphor with begetting: things reproduce after their own kind. Most English words from this root emphasize only having something in common, but the notion of begetting is still there, just a little below the surface: a generation, as in "the 1960s generation", is all the people born around the same time, seen as part of a sequence of generations each giving birth to the next. The word genus can also mean any sort of group or kind with something in common, but in Latin the metaphor with birth and family is still clear.
The suffix -osus makes an adjective indicating having an abundance of the noun it's based on, similar to the Anglo-Saxon suffix -ful. For example, dolor is pain or sadness; dolorosus means painful or sorrowful. (In English, -osus appears as -ous; e.g. "cavern" and "cavernous", "luxury" and "luxurious".) When added to some nouns, like genus, it brings a positive connotation: if you are generosus, then you come from good stock, you are well-bred. As in English, the Latin word is also used to mean giving freely, since a noble family properly shows largesse, but in Latin this is an extended sense; the primary meaning is being born to noble ancestry—"genetic" superiority, you could say.
It should come as no surprise, then, that generosus is a common word in heraldic mottos, where it is an adjective for "gentleman". The exact meaning of this varied a lot over the centuries, as the criteria for what entitled you to the rank of gentleman shifted, but the basic connotation remained the same. Putting generosus onto your coat of arms asserts that you come from a long line of respectable ancestors even if you don't have a title of nobility. Here's a dictionary of heraldry from 1828 that says some more about this. And notice the sentence in the middle of the bottom row of the picture below: Generoso germine gemmo: "From an excellent bud I sprout." Well, "excellent" doesn't carry the precise meaning, of course, but you get the idea.
Source: The English Gentleman by Richard Braithwait (1630).
The noun virtus = vir + -tus. This one is easier to explain. Vir means man, and -tūs forms a noun meaning the essential quality of the first part of the word, as iuvenis, "a young person", and iuventus, "youth". The path from Latin virtus to contemporary English "virtue" is a long one, involving trips through Christianity and supernaturalism, described a little more in the answers to this question. The main thing to know is that in Latin, virtus means manliness, especially strength, courage, honor.
In the Tusculan Disputations 2.15–16 someone tells Cicero that pain is the worst evil. Cicero asks "Worse than dishonor?" The other person immediately concedes that he was wrong, embarrassed to have been refuted so quickly. A little later Cicero says:
Ergo id quod natura ipsa et quaedam generosa virtus statim respuit, ne scilicet dolorem summum malum diceres oppositoque dedecore sententia depellerere, in eo magistra vitae philosophia tot saecula permanet?
So, what nature itself, and a certain [worthy, innate uprightness?], immediately spat back out—your saying that pain is the worst evil, a sentiment you were driven away from when it was contrasted with dishonor—the mistress of life, philosophy, is going to maintain that for so many ages?
I'm not sure that any English words can satisfactorily translate Cicero's use of the phrase, but hopefully the meaning is starting to become clear.
A smoother translation
So finally here is a less clumsy translation of the motto:
Gentlemanly courage fears nothing.
I think this conveys less of the spirit of the motto than the clumsy version did, but it's certainly less jarring if you need a translation that you can use in English prose. I'm avoiding the word "noble" because, in the context of coats of arms, this suggests having a peerage—that is, being a peer of the royalty. A peerage might be hereditary, but originally it is earned: it is conferred by a monarch in honor of a great deed or service. Hence fit nobilis, nascitur generosus: nobility is made, gentility is born (more here). Thus the two are quite distinct, at least in the feudal worldview that gave rise to coats of arms and heraldic mottos.
Note that I am not an expert on heraldry. A while back, I happened to get curious about generosus, since it wasn't being used in Latin the way I expected—whence the explanation above. And yesterday, your question triggered me to google a bunch about the word's use in coats of arms.