T.G.I.F VS S.H.I.T.

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 possibly, "T-G-I-F."
The man smiled back at her and once again said , "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."