ДУБ _|_ OAK / DUB [dup] | ДУБИТЬ _|_ TO BARKEN / DUBIT [dʊˈbʲitʲ]
Hint with an alternative definition: it is a dour & big tree (think about an old & big oak from Dublin, the Druids’ tree of doom & being), it’s like a super-boss in a wood, burr or scrub, its branch, bough are good to use as a wooden club to beat or dub someone.
Дубить: to double it = make hard and solid like an oak tree or to daub with a liquid from the bark of a tree
Hints with indirect antonym + context: dupe (as fool) > blockhead > a big tree with a hole (oak)
Mental visualization: Д (oak trunk with roots) + У (oak branches) + Б (oak crown with one branch at the top)