If people start calling you "Mac" because you like to eat macaroni and cheese for every meal, then you not only have a strange diet, but you also have a sobriquet — in other words, a nickname.
Sobriquet is derived from the French word soubriquet. Sobriquets are often but not always humorous, so in order to pronounce this word, you might want to remember that the last syllable rhymes with play. Sobriquets are usually given to you by other people, but you can choose one for yourself if you want. If your name is Jennifer and you tend to introduce yourself as Jen, this also qualifies as a sobriquet.