be sharp enough to cut glass

1. To have a very prominently defined point or edge. Often said of physical features. It used to be the case that your cheekbones had to be sharp enough to cut glass if you wanted to be a model, but those standards are starting to change. The movie star has such strong features, with piercing blue eyes, an impressive physique, and a jawline that is sharp enough to cut glass. Because you have features that are sharp enough to cut glass, you're pretty much a dream come true for a makeup artist like me!

2. To be harsh, bitter, or hurtful, as of one's voice, words, or tone. I knew Mom was furious as soon as she opened her mouth. Her tone was sharp enough to cut glass. I hate working with that guy. His so-called "constructive criticism" is always sharp enough to cut glass. I really don't think that constantly correcting your employees with words that are sharp enough to cut glass will endear you to them.