The term 'THAT'll be the day!' is an expression of disbelief or incredulity, usually in response to something THAT seems impossible. The crossword solutions for this phrase are HAH (3 letters), ASIF (4 letters), IBET (4 letters), WHPIGSFLY (9 letters) and IWOULDNTBETONIT (15 letters). HAH stands for 'Ha Ha', which is a laugh indicating amusement or derision. ASIF stands for 'As If', which is used to express disbelief or doubt. IBET stands for 'I Bet', which is used to express a challenge or a wager. WHPIGSFLY stands for 'When Pigs Fly', which is an expression of extreme skepticism. Finally, IWOULDNTBETONIT stands for 'I Wouldn't Bet On It', which is another way of expressing doubt or disbelief. All of these phrases are used to express disbelief or doubt, and when combined, they form the phrase 'THAT'll be the day!', which is used to express extreme skepticism.