This is what I've found on Etymonline real estate
REAL (adj.) early 14c., "real, actually existing, true;" mid-15c., "relating to things" (especially property), from O.Fr. reel, from L.L. realis "actual," from L. res "matter, thing," of unknown origin. Meaning "genuine" is recorded from 1550s; sense of "unaffected, no-nonsense" is from 1847.
Real estate is first recorded 1660s and retains the oldest English sense of the word.
So, according to this source, "real" means "existing, true" as per the Latin origin, but then even the later meaning of "relating to things" seems to derive from the same root (through French).
Minor addition :
in Italian the term "reale" has the same double meaning it has in Spanish (royal and real), whereas this is not true for French (royal and réel), although the three languages all derive from Latin...