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1.
Anytime the penis is only partly erect. Often used sarcastically when there is no abnormal blood flow down below.
"Did you just see how fast she ate that hot dog?"
"Yeah, she just gave me a half stock"
by Brock Stokley April 13, 2006
 
2.
semi-erect or semi-flacid. However you want to look at it... I choose semi-erect, as that seems more positive.
"Man, I'd fuck that shit out of that bitch, but I'm halfstock."
by Beil February 13, 2005
 
3.
When you're half way down from an erection. Contrast to half mast being half way up to an erection.
I finished fucking her and now I'm half stock.
by VTI December 02, 2006