if need be

Need is a noun, be a subjunctive. Perhaps an alteration of Old English phrases like ġif þearf bēo, literally "if need be". That phrase would have been completely unidiomatic, equivalent to "if a need exists" or "if there is a need". This is because (1) Old English had no indefinite article, (2) the word for "to be" was also the default word for "to exist" and translates the modern phrase "there is", and (3) the subjunctive mood was much more common and often translates the Modern English indicative.