Cousu De Fil Blanc

The expression cousu de fil blanc literally means stitched with white thread.  It is used to describe something so obvious, transparent, or easily seen through, that it practically has a neon sign blinking above it.  Have you heard the story about the student whose dog ate their homework? That’s a perfect example of something that's cousu de fil blanc. The student's excuse is so transparent that it doesn't fool anyone.  Even the family dog is side-eyeing them.  

A similar expression you might hear is C'est gros comme le nez au milieu du visage, which means it's as obvious as the nose in the middle of your face.  Both phrases convey the idea of something being obvious or easily noticeable.  However, cousu de fil blanc implies deceit or trickery, while the other expression is more general in its indication of obviousness.

Origin

The expression comes from the simple fact that, as any self-respecting seamstress knows, for a seam to be as discreet as possible, it should be stitched with thread that matches the fabric perfectly. Otherwise, it stands out like a golden retriever at a cat convention.  It’s easy to see why this metaphor works. The white thread is like those “big strings” that make any sneaky plot as obvious as white stitches in a black suit.  The idiom has been around since the late 16th century, proving that some things never change - like our talent for bad hiding!

Examples

L'intrigue du film était cousue de fil blanc, on devinait facilement le dénouement.
The plot of the movie was so obvious that you could easily guess the ending.

Son excuse était cousue de fil blanc, tout le monde a compris qu'il mentait.
His excuse was so transparent that everyone understood he was lying.