Two out of fifty?

• The only grading letters that appear anywhere in this puzzle (other than the present clue)
• Two-thirds off?
• Pair of earmuffs?
• Flunking grades
• Affair twosome?
• 4-point Scrabble tiles
• Letters starting many military plane names
• Insufferable pair?
• Pair in Banff?

Wiktionary Word definitions in Wiktionary
n. (plural of ef English), the name of the letter F.