Keyhole
Keyhole turns up in 2 fields: woodworking, shop slang. Here is what it means to the people in each one.
Keyhole in woodworking
A shaped slot or mortise cut into a timber to receive a locking wedge or key.
A specialized recess, mortise, or slot cut into intersecting timbers to accommodate a wooden key or wedge. Once the joint is assembled, driving the key into the keyhole locks the pieces tightly together, drawing the shoulders flush without the need for glue or metal fasteners. It is a hallmark of traditional timber framing and knockdown furniture.
"Cut the keyhole slightly tapered so the wedge pulls the whole joint tight as you drive it home."
Keyhole in shop slang
To drill or cut a hole that becomes elongated or asymmetrical instead of perfectly round.
In machining and fabrication, this defect occurs when a drill bit wanders off center or chatters during the cut. It can also happen in welding when the weld pool collapses and leaves an oblong gap. The resulting flaw ruins the tolerance of the workpiece and usually requires filling, re-drilling, or scrapping the part entirely.
"Feed that bit slower through the sheet metal or it will keyhole and ruin the mounting bracket."