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1.
The act of having sex without a full erection. Usually occurs when one is conciously grossed out by the person he is having sex with, but engaging her in sexual intercourse anyway just for the sake of busting a nut or due to sheer boredom. Almost always is sensationaless and unsatisfying, and prevalence almost always increases when drugs and alchohol are involved.
"Yo you fuck that bitch last night"?
"Yea, it was awful though, she was so doggish I was half mastin but I went through with it anyway cuz I'm a trooper."
by c slappa March 25, 2010
 
2.
Erect enough to cause embarrassment
After he saw that new Chris Brown video, Eric Cano was half mastin'..............ya heard me?
by C_Los July 14, 2006