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1.
1. Noun. A guitar amp consisting of an amp head and a 4x12 speaker cab.

2. Noun. A young guitar player who thinks that they know more than they do.
1. Hey dude! I just got a Mesa-Boogie Dual Rectifier Half-Stack!

2. Young Musician: Yeah, Line 6 Is way better than Marshall.

Experienced Musician: Kid, you've got a long way to go before you can say that.
by Kurt_F July 15, 2008
 
2.
A semi-erect penis.
"Yo, that phat ugly bitch aint even gone give me no none of a mo-fucking half-stack , bitch is straight stank-nasty."
by sand-hole February 16, 2009
 
3.
Derived from the address "555 1/2 W Holland St.", along with a half stack amplifier metaphorically speaking, the Halfstack is the most amazing bachelor pad in the southeast region of South Dakota. Business offices below the apartment share the address "555 W Holland St.," thus the partial "1/2" comes into play. Apartment tenants go by the names "Josh" and "Kyler."
Dude, we gotta go to the Halfstack for a good jam session!

In verb form:
Joe is Halfstacking it up with his buddies.
by Joshua Harrison Ford October 15, 2010