You mean in its entirety, in its whole whole, entirely, completely. All.
In the whole of something, completely. The original phrase seems to be "out of the Way" ("from End to end"), but to refer to animals and even people, the popular ingenuity modified it with this version Refranesca to say "from head to tail".
It is a colloquial expression that means: from end to end, from span to span; side-by-side and from top to bottom.