a figment of (your) imagination

something created by your mind.

I thought I saw someone standing in the shadows, but it was just a figment of my imagination.

• someone's hackles rise
• someone's hands are tied
• someone's head on a plate
• someone's head on a platter
• someone's heart is in boots
• someone's heart is in mouth
• someone's heart is in their boots
• someone's heart is in their mouth
• someone's heart is in the right place
• someone's heart isn't in
• someone's heart isn't in something
• someone's heart misses a beat
• someone's heart sinks
• someone's heart skips a beat
• someone's mask slips
• someone's name is mud
• someone's number is up
• someone's other half
• someone's party piece
• someone's paths cross
• someone's pet hate
• someone's place in the sun
• someone's right-hand man
• someone's right-hand woman
• someone's stamping ground