two words that are related in descent are said to be "cognates". This term is particularly likely to be used for two words in different languages with shared descent. Words which come from the same ultimate source may be said to share a "common root" or "common stem" and the source word may be called the "root" or "stem" or "base".
Note that superficial similarities in word ending are often false cognates words that seem to be related, but in fact are not. Note that actual cognates, particularly when the common root is distant, are often not obviously similar in form. For example "fist" is ultimately believed to derive from an Indo-European root "*penkwe-" meaning hand, but the source of many words meaning "five" for the five fingers. This root is the source of, among others: cinquecento; cinquefoil; fifteen; fifth;; finger; fist; five; foist; parcheesi; penta-; pentacle; pentad; Pentateuch; Pentecost; pentagon; pentagram; Pompeii; Punjab; punch; quintain; quintet; quintile; quintessence; and various others
According to tymonline confuse is indeed cognate with refuse, and also with such words as: alchemy; chyle; chyme; confound; foundry; funnel; fuse; fusion; futile; futility; geyser; gush; gut; infuse; ingot; parenchyma; perfuse; perfusion; profuse; refund; and various others, from a root "*gheu-" meaning "pour"
Note that in linguistics, a word shown starting with an asterisk has been deduced to exist, but has not actually been found in written texts.