perdre le fil

You use it just like the English phrase "to lose my train of thought." It is a verb phrase, so you conjugate perdre based on who is speaking. For example, J'ai perdu le fil means "I lost my thread." You can use it when you get interrupted by a loud noise. You can use it if your friend starts talking over you. It is perfect for those "wait, what was I saying?" moments.

Use this during a presentation if you get nervous. Use it when you are telling a long, winding story at a bar. It is great for apologizing after a long tangent. If someone asks you a question and you forget your original point, say it. It makes you sound very natural and human. It shows you are engaged but perhaps a bit tired or distracted.

Do not use this if you are literally under pressure at work. Even though some translations suggest pressure, it specifically refers to the flow of thought. If your boss is yelling about a deadline, do not say je perds le fil. That would mean you forgot what the deadline was for. Also, avoid it in very short, simple commands. It requires a bit of context to make sense.

This phrase comes from the ancient Greek myth of Ariadne. She gave Theseus a ball of thread to find his way out of the Labyrinth. If he lost the thread, he was doomed to wander forever. Today, we use it for conversations instead of monsters. It implies that a good conversation is a path we walk together. If you drop the thread, the connection is temporarily broken.

You might hear perdre le fil de la conversation for more detail. Some people say j'ai un trou de mémoire for a total memory blank. If you want to be more casual, you could say je ne sais plus où j'en étais. However, perdre le fil remains the most elegant and common way to describe a mental slip. It is a classic for a reason.