Do you mean to ask if it is used in the English language? The answer is that it is.
Tubby was first used in 1835 to mean “shaped like a tub”. The more modern definition of “fat” originated in 1891. The word tub was first used in the 14th century and came from either the Middle Low German, Middle Dutch, or Middle Flemish tubbe.
Tub is generally pronounced (using the International Phonetic Alphabet) as /tʌb/. Tub has one syllable and rhymes with club and pub. While /tʌb/ is considered the standard pronunciation, there may be dialects or accents that pronounce the word differently.
If you want to learn more about the International Phonetic Alphabet, you can find a guide here: International Phonetic Alphabet and Phonemic Alphabets