"Undoubtably" is not an official word. But people still use it. The artist probably used it because it rhymes and matches rhythm of the lyrics.
"Undoubtedly" is more accepted in formal writing.
If you want to sound more intelligent, "indubitably" is your best option.
I hope this is helpful.
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栗子的知乎答案索引：栗子樹
栗子的微信公眾號：E-Speller 或者 栗子英文 （只有精彩的原創）
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檢索水平有待加強。
這個詞可以有，不過一般被認為不標準、不正規，但現代英語中確實有人用，甚至學界也有用的：
undoubtably - Wiktionary
"Undoubtably" is considered to be nonstandard English by some authorities (for example, Garner"s Modern American Usage (2009)), and the term is seldom found in modern literary writing.[1] The Oxford English Dictionary provides no examples of usage after 1513 and characterizes "undoubtably" as "? Obs.," wondering whether the term is obsolete.[2] Nevertheless, many examples of its usage can be found in 20th- and 21st-century popular English and in contemporary academic journals.
儘管被認為不正規，可以用正規的 undoubtedly 代替，但也有人認為兩個詞有細微的差別：
Undoubtably vs. undoubtedly
Undoubtably and undoubtedly are both well-formed words with clear, distinct meanings, yet the former is often used in place of the latter, giving rise to the mistaken belief that undoubtably is always wrong. It』s not. The distinction between them can be subtle; something that is undoubted is not doubted, while something that is undoubtable is not capable of being doubted. The two meanings often overlap, which is why mixing up the two words is often not a serious error, yet the difference is occasionally important. For example, one might claim that in his or her opinion the Beatles are undoubtably the most influential rock band in history, yet we probably couldn』t say they are undoubtedly so, because even one who believes the Beatles to be the most influential band must acknowledge that there are rock critics who doubt this.
