to avoid showing any emotion, esp. amusement.

I thought it would be funny to tell him he failed the course, but it was hard to keep a straight face when I was talking to him.

• fall through
• fall through the cracks
• fall through the floor
• fall to pieces
• false alarm
• false dawn
• false economy
• false start
• Familiarity breeds contempt.
• family man
• Famous last words.
• fancy-pants
• fan out
• fan the flames
• fan the flames of
• fan the flames of something
• far and wide
• far be it from me
• far be it from me to
• far be it from me to do something
• far cry from
• far cry from something
• far from
• far from it
• Far from it.