Green’s Dictionary of Slang

1. (US prison) an inmate entrusted with keeping a gang’s supply of drugs.

1992		Bentley & Corbett Prison Sl. 42: Much of the drug traffic in prison is controlled by the various gangs. [...] consequently, gang members are always under suspicion and frequently searched. Therefore, it is important for gangs to have someone, who is not directly associated with them and is not a drug user, to keep drugs for them until they are sold. Such a person is referred to as a holder.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 88/2: holder n. an inmate in possession of drugs or other contraband, one who is holding.

2. as sense 1, but for street drug sales.

2014

Ruderman & Laker Busted 46: Above the corner boys were the holders, or guys who stashed the dope.