"open wooden vessel made of staves," late 14c., from Middle Low German, Middle Dutch, or Middle Flemish tubbe, of uncertain origin. Related to Old High German zubar "vessel with two handles, wine vessel," German Zuber. Considered to be unrelated to Latin tubus (see tube (n.)); one theory connects it to the root of two based on the number of handles. Also 17c. slang for "pulpit;" hence tub-thumper (1660s) "speaker or preacher who thumps the pulpit for emphasis."
adjective suffix, "full of or characterized by," from Old English -ig, from Proto-Germanic *-iga- (source also of Dutch, Danish, German -ig, Gothic -egs), from PIE -(i)ko-, adjectival suffix, cognate with elements in Greek -ikos, Latin -icus (see -ic). Originally added to nouns in Old English; used from 13c. with verbs, and by 15c. even with other adjectives (for example crispy).
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<a href="https://www.etymonline.com/word/tubby">Etymology of tubby by etymonline</a>
Harper, D. (n.d.). Etymology of tubby. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/tubby