Your mom is so fat, she starts with the letter O in the alphabet because O.B.C.D. (Obesity).
Alphabet Obesity
Categories
Explanation
Experiment
Explain Bear
Alright listen up buttercup. This joke suggests someone's mom is super obese! How obese? So obese that she represents the first letter in "obesity". The joke implies someone's mother is defined by their obesity. Bet you never thought of that.