Crebain is a plural Elvish word, with the presumed singular form being craban. Unfortunately, no etymological explanation of this name is attested, though craban can probably be taken simply as the Sindarin word for 'crow'. There are very few Elvish words that are even remotely plausible as an ultimate origin of the name, with perhaps the old word rhafn, 'wing', being the only realistic possibility in the known Elvish corpus.

It is perhaps notable that rhafn is somewhat similar to English 'raven', and it has been speculated that there may be a connection via Old English hræfn. This is not only the origin of the modern English word 'raven', but is also distinctly reminiscent of Elvish rhafn, so we might perhaps speculate that the word craban was inspired by Anglo-Saxon hræfn, with Tolkien devising an alternative etymology to conceivably connect the word to Elvish word Crebain. (The potential link to Old English hræfn is suggested in Jim Allan's An Introduction to Elvish, quoted in The Lord of the Rings: A Reader's Companion.)

Crows and ravens are of course different - but related - kinds of birds, but they are similar enough that a potential linguistic connection like this is not entirely unrealistic.