Someone who sucks at whatever it is they are doing. It originated sometime in the 1800's in the world of bricklaying. One who spent too much time on one brick or block, tapping on it too much with his hammer, would be sarcasticly called a "shoemaker"; because shoemakers "tap" endlessly with their hammers. Over time, this was reduced to "shoe", and eventually a "boot". Boot now refers to any bricklayer who is sloppy, apothetic, or just overall bad at laying brick. It is a very strong term. Calling a bricklayer a boot is the worst thing you could possibly say to him, since it is the ultimate insult to their craftsmanship. All bricklayers know this. But in recent years it has spread past the world of masonry and now it seems to pertain to anyone who sucks at something that they are doing; or trying to do.
someone is nailing and hits their finger.
"AAAHH, you BOOT!"
Man, that guy cant nail for shit.. he's a boot!
Man, look at that wall out of level. They must have been real BOOTS!
Only a boot would lay that garbage.