When someone prevents you from getting an erection, or does something that completely turns you off.
Just like a lumberjack cuts down a huge tree, it can be compared to someone killing your wood
Similar to a cock-block but worse; it could ruin several moments instead of just one.
Bro1: " Dude, last night my roommate walked in on my girlfriend and I making out. The date was ruined and now my girlfriend is afraid of my roommate walking in on us."
Bro 2: "What a lumberjack!!!!"