I say it means "Hidden".
"That which is not obvious" - as is the material world, or the mundane knowledge and activities associated with it which are visible to all - "and that which must be sought after."
Definitive dictionaries are not always helpful.
If you are not a bricklayer then certain behaviours and practices employed by the bricklayer may be regarded as esoteric. Even within the bricklaying class there are those who have developed their skills and knowledge beyond some others who have merely learnt a trade and practice it at basic levels. To these latter, some aspects of bricklaying could be seen as esoteric.
The road-sweeper, like the swordsman, or mathematician, may attempt to perfect his art, but it must be pursued consciously.
We do not have to be thinking only of such as Pythagoras. He sought Perfection through the Mystery schools of Greece and Egypt, according to his nature and abilities, but the road-sweeper, with a different nature and other abilities, may just as successfully seek perfection in his own art. First, of course, he must make it his art.
To think only of such as Pythagoras as being involved in esoterism is to deny the possibility of our own advancement by seeking Perfection even in the daily drudgery of the housework we may may have resigned ourselves to. Thus the adage; "Seek perfection in everything you do."
As the Messenger of God informed us, "There are as many paths to God as there are souls of men." If this were not the case, and certainly only one of us could ever be Pythagoras, the rest of us are doomed.
Musashi.