a fine line
a fine line
A very narrow division between two deceptively similar things, one of which is worse than the other. As a new parent, you learn that there's a fine line between looking out for your child's welfare and being overprotective. A: "I only borrowed your sweater, I swear!" B: "Well, it's a fine line between borrowing and stealing, isn't it?"
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
a fine line between something
COMMON If there is a fine line between two different activities or situations, there is a point at which they are very similar, often when one activity or situation is acceptable, and the other is not. There is a fine line between being nicely looked after and being fussed over too much. A new exhibition explores the fine line between genius and insanity. Note: You can also talk about a thin line between two things or a narrow line between two things. There is a thin line between being a good player and being one of the best. There's a narrow line between being interested and being nosy. Compare with walk a fine line between something.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012