Who is supremely great. The one and only being, whose existence is free of time, place and similarity to any of the creatures.
“Allah doth know what every female (womb) doth bear, by how much the wombs fall short (of their time or number) or do exceed. Every single thing is before His sight, in (due) proportion. He knoweth the Unseen and that which is open: HE IS THE GREAT, the most High. It is the same (to Him) whether any of you conceal his speech or declare it openly; whether he lie hid by night or walk forth freely by day.” (13:8.10) The Holy Quran
From the root k-b-r with the classical Arabic connotations: to be great in rank, dignity, nobility, majesty, to be most knowing to have rights above all others.