it means you can eat off the floor with confidence. Just like an old proverb about a "speck of dust" on the ground. overbrimming means excessively over confident. Proverbial = a well known idiom of way of speaking. It seems like the writer is using overbrimming in a sort of reverse way. Mrs Cuthbert was meticulously clean always sweeping her back yard. "a SPECK" = a very small powdery usually dust substance or a very tiny stain on a surface. Example sentence ="I want you to dust of every Speck of dust from the top of the wardrobe" From an internet search another example sentence "Billy leaned forward and brushed a speck of dust off his shoes". I have forgotten the proverb to quote for/to you. Or there may not be one, it may just be a way of speaking, not exactly an idiom.

There is a fairly rare proverb that says "we must eat a peck of dirt before we die". A peck is about 9 liters. So, every human will eat about 9 liters of dirt in their life. The proverb is usually used to remind people that there is always a small amount of unpleasantness mixed with the good things in life. If you eat meals off the ground, you would expect that you would quickly exceed your 9 liters--your peck of dirt would overflow. However, here, the ground was so clean that you could eat on it without eating any dirt.

Hi! This is an expression that means that something is very clean. In this case Mrs. Rachel thinks the porch is very clean. It is so clean that someone could "eat a meal off it" and not have any dirt on their meal. Hope this helps!