From p353 of The Psychology of Day-Dreams by Dr J. Varendonck, published in 1921:
Like nocturnal dreams, day dreams betray preoccupations with unsolved problems, harassing cares, or overwhelming impressions which require accommodation, only their language is not as sibylline as that of their unconscious correspondents…
But they all strive towards the future; they all seem to prepare some accommodation, to obtain some prospective advantage to the ego; in fine, they are attempts at adaptation: such is their biological meaning. They complete the functions of consciousness without our mental alertness.
Varendonck was attempting to apply Freud’s theory of dreaming to daydreams, and, as was customary at the time, largely based his theories on ideas generated from his own daydreams.
I had to look up ‘sibylline’. Apparently it relates to the Sibylline oracles and in this context it means ‘knowledge giving’.