Where does “reg” come from?

Reg is an English back-formation from regolith, itself formed from Greek rhêga (break) and -lith (stone).

reg (English): A regular; A regulation; registrar

Definitions

Step	Language	Word	Meaning
1	English	regolith	The layer of loose rock, dust, sand, and soil,...
2	Ancient Greek	ῥῆγος	blanket, coverlet; cover, cushion

reg — Etymology, Origin & Word History | Etymology Explorer