I believe it to be an euphemism similar to how today one might say something like gazillions for a large number.
In a society that is mostly innumerate as well as illiterate, where neither pencils nor paper exist and both slateboards and chalk are fragile and rare, the scale of numbers readily accessible to the common population are much smaller than today. ...
There seems to be a bit of pushback on Pieter's (correct) answer, so perhaps a bit more detail is in order.
It is not at all uncommon in languages to have words that, while technically a specific number, are usually used just to indicate an unspecified large amount. One of the technical terms for this is non-numerical vague quantifiers.
The most well-known ...
Biblically, Forty is a number associated with testing and trials
Jesus wandered in the wilderness 40 days, Israel wandered in the desert for 40 years etc.
It's a number that indicates enough time passed for God to achieve a goal. Fullness of time.
Motivation for the answer
I'll take a literary approach, aiming less at the surrounding historical context and more at an internal analysis of what "significance" the document's authors intended to convey.
However, rereading your question and David Robinson's comment, I see that you're not asking about this significance so much as the motivation to use "40"...
There's more than one:
Isaiah (chs. 30-31) criticizes the Israelian and Judean rulers of his time for trying to ally with Egypt against Assyria, and predicts that these efforts will end in failure.
Hosea, a contemporary of Isaiah, also criticizes them for vacillating between Assyria and Egypt (7:11 and 12:2).
Jeremiah (about a century later, when Assyria ...
It is still not very much known about the text in question. But for a direct parallel in anything 'bible' it seems safe to say: no.
kw 333 imhw imhw (PT 235 §239a) […] suggesting that the phrase imhw may be 'm hwy 'mother of snake.' Recalling that Egyptian 3 was originally used to render Semitic r, […] further that 333 was the name Rir-Rir, a ...