A business man got on an elevator.
When he entered, there was a blonde already inside who greeted him with a bright, "T-G-I-F".
He smiled at her and replied, "S-H-I-T".
She looked puzzled and repeated, "T-G-I-F", more slowly.
He again answered, "S-H-I-T". The blonde was trying to keep it friendly, so she smiled her biggest smile, and said as sweetly as possible, "T-G-I-F"
The man smiled back to her and once again "S-H-I-T". The exasperated blonde finally decided to explain. "T-G-I-F" means 'Thank Goodness It's Friday.' Get it, duuhhh?"
The man answered, ''S-H-I-T" means 'Sorry, Honey, It's Thursday'".
In e-mail from my aunt Margaret... unknown original source.