Green’s Dictionary of Slang

1. (US milit.) a regular soldier.

1907		C. M’Govern By Bolo and Krag 186: Guess them regs. wants to show off that they ain’t afraid to go around without their guns like us hobo Volunteers.
1932		B.M. Harvey Me and Bad Eye and Slim 191: Corp. Quinn, a old reg we call Mother Quinn.

2. (drugs) regular strength marijuana.

1973

Wesson & Smith Uppers and Downers.

3. a regular, e.g. regular customer.

2001		N. Barlay Hooky Gear 225: The baths is openin an a couple regs is already waitin.
2001		K. Sampson Outlaws (ms.) 35: And the Billies are mainly regs and all, too. Even if it’s just an auld dear that wants taking home from the Asda twice a week.

4. (Irish) registration.

2021

Breen & Conlon Hitmen 66: When the car was turning I [saw] some of the reg.