be/lie at the bottom of (something)

to be the real reason for something unpleasant.

I don't know for certain why she dislikes you, but I suspect jealousy is at the bottom of it.

• flog a dead horse
• flog something to death
• flog to death
• floor it
• flotsam and jetsam
• flunk out
• flunk out of school
• flush out
• flush out someone
• flush out something
• fly blind
• fly-by-night
• fly by the seat of pants
• fly by the seat of your pants
• flying high
• fly in the face of
• fly in the face of something
• fly in the ointment
• fly into a rage
• fly off the handle
• fly-on-the-wall
• fly on the wall
• fly the coop
• fly the flag
• fly the nest